#Aristotle: Logic

Aristotelian logic, after a great and early triumph, consolidated its position of influence to rule over the philosophical world throughout the Middle Ages up until the 19th Century.  All that changed in a hurry when modern logicians embraced a new kind of mathematical logic and pushed out what they regarded as the antiquated and clunky method of syllogisms.  Although Aristotle’s very rich and expansive account of logic differs in key ways from modern approaches, it is more than a historical curiosity.  It provides an alternative way of approaching logic and continues to provide critical insights into contemporary issues and concerns.  The main thrust of this article is to explain Aristotle’s logical system as a whole while correcting some prominent misconceptions that persist in the popular understanding and even in some of the specialized literature.  Before getting down to business, it is important to point out that Aristotle is a synoptic thinker with an over-arching theory that ties together all aspects and fields of philosophy.  He does not view logic as a separate, self-sufficient subject-matter, to be considered in isolation from other aspects of disciplined inquiry.  Although we cannot consider all the details of his encyclopedic approach, we can sketch out the larger picture in a way that illuminates the general thrust of his system.  For the purposes of this entry, let us define logic as that field of inquiry which investigates how we reason correctly (and, by extension, how we reason incorrectly).  Aristotle does not believe that the purpose of logic is to prove that human beings can have knowledge.  (He dismisses excessive scepticism.)  The aim of logic is the elaboration of a coherent system that allows us to investigate, classify, and evaluate good and bad forms of reasoning.

Table of Contents

1. The Organon
2. Categories
3. From Words into Propositions
4. Kinds of Propositions
5. Square of Opposition
6. Laws of Thought
7. Existential Assumptions
8. Form versus Content
9. The Syllogism
10. Inductive Syllogism
11. Deduction versus Induction
12. Science
13. Non-Discursive Reasoning
14. Rhetoric
15. Fallacies
16. Moral Reasoning
17. References and Further Reading
    1. Primary Sources
    2. Secondary Sources

1. The Organon

To those used to the silver tones of an accomplished writer like Plato, Aristotle’s prose will seem, at first glance, a difficult read.  What we have are largely notes, written at various points in his career, for different purposes, edited and cobbled together by later followers.  The style of the resulting collection is often rambling, repetitious, obscure, and disjointed.  There are many arcane, puzzling, and perhaps contradictory passages.  This problem is compounded by the abstract, technical vocabulary logic sometimes requires and by the wide-ranging scope and the scattered nature of Aristotle’s observations.  Some familiarity with Greek terminology is required if one hopes to capture the nuances in his thought.  Classicists and scholars do argue, of course, about the precise Greek meaning of key words or phrases but many of these debates involve minor points of interpretation that cannot concern us here.  Aristotle’s logical vocabulary needs to be understood within the larger context of his system as a whole.  Many good translations of Aristotle are available.  (Parenthetical citations below include the approximate Bekker number (the scholarly notation for referring to Aristotelian passages according to page, column, and line number of a standard edition), the English title of the work, and the name of the translator.)

Ancient commentators regarded logic as a widely-applicable instrument or method for careful thinking.  They grouped Aristotle’s six logical treatises into a sort of manual they called the Organon (Greek for “tool”).  The Organon included the Categories, On Interpretation, the Prior Analytics, the Posterior Analytics, the Topics, and On Sophistical Refutations.  These books touch on many issues: the logical structure of propositions, the proper structure of arguments (syllogisms), the difference between induction and deduction, the nature of scientific knowledge, basic fallacies (forms of specious reasoning), debating techniques, and so on.  But we cannot confine our present investigations to the Organon.  Aristotle comments on the principle of non-contradiction in the Metaphysics, on less rigorous forms of argument in the Rhetoric, on the intellectual virtues in the Nicomachean Ethics, on the difference between truth and falsity in On the Soul, and so on.  We cannot overlook such important passages if we wish to gain an adequate understanding of Aristotelian logic.

2. Categories

The world, as Aristotle describes it in his Categories, is composed of substances—separate, individual things—to which various characterizations or properties can be ascribed.  Each substance is a unified whole composed of interlocking parts.  There are two kinds of substances.  A primary substance is (in the simplest instance) an independent (or detachable) object, composed of matter, characterized by form.  Individual living organisms—a man, a rainbow trout, an oak tree—provide the most unambiguous examples of primary substances.  Secondary substances are the larger groups, the species or genera, to which these individual organisms belong.  So man, horse, mammals, animals (and so on) would be examples of secondary substances.  As we shall see, Aristotle’s logic is about correctly attributing specific properties to secondary substances (and therefore, indirectly, about attributing these properties to primary substances or individual things).

Aristotle elaborates a logic that is designed to describe what exists in the world.  We may well wonder then, how many different ways can we describe something?  In his Categories (4.1b25-2a4), Aristotle enumerates ten different ways of describing something.  These categories (Greek=kategoria, deriving from the verb to indicate, signify) include (1) substance, (2) quantity, (3) quality, (4) relation, (5) where, (6) when, (7) being-in-a-position, (8) possessing, (9) doing or (10) undergoing something or being affected by something.  In the Topics (I.9, 103b20-25), he includes the same list, replacing “substance” (ousia) with “essence” (ti esti).  We can, along with Aristotle, give an example of each kind of description: (1) to designate something as a “horse” or a “man” is to identify it as a substance or to attribute an essence to it; (2) to say that the wall is four feet long is to describe it in terms of quantity; (3) to say that the roof  is “white” is to ascribe a quality to it; (4) to say that your weight is “double” mine is to describe a relation between the two; (5) to say something happened in the market-place is to explain where; (6) to say it happened last year is to explain when; (7) to say an old man is sitting is to describe his position; (8) to say the girl has shoes on is to describe what she possesses; (9) to say the head chef is cutting a carrot with a knife is to describe what he is doing; and finally, (10) to say wood is being burned in the fireplace is to describe what it means for the wood to undergo burning or to be affected by fire.  Commentators claim that these ten categories represent either different descriptions of being or different kinds of being.  (To be a substance is to be in a certain way; to possess quantity is to be in a certain way; to possess a quality is to be in a certain way, and so on.)  There is nothing magical about the number ten.  Aristotle gives shorter lists elsewhere. (Compare Posterior Analytics, I.22.83a22-24, where he lists seven predications, for example).  Whether Aristotle intends the longer lists as a complete enumeration of all conceivable types of descriptions is an open question.  Scholars have noticed that the first category, substance or essence, seems to be fundamentally different than the others; it is what something is in the most complete and perfect way. Read More.

#Contradiction

Laurence R. Horn <laurence.horn@yale.edu>

First published Wed Jun 28, 2006; substantive revision Wed Aug 29, 2018

Do I contradict myself?
Very well, then, I contradict myself.
(I am large, I contain multitudes.)
   —Walt Whitman, “Song of Myself”

Vorrei e non vorrei. 
   —Zerlina, “Là ci darem la mano”, Don Giovanni

This entry outlines the role of the law of non-contradiction (LNC) as the foremost among the first (indemonstrable) principles of Aristotelian philosophy and its heirs, and depicts the relation between LNC and LEM (the law of excluded middle) in establishing the nature of contradictory and contrary opposition. §1 presents the classical treatment of LNC as an axiom in Aristotle's “First Philosophy” and reviews the status of contradictory and contrary opposition as schematized on the Square of Opposition. §2 explores in further detail the possible characterizations of LNC and LEM, including the relevance of future contingent statements in which LEM (but not LNC) is sometimes held to fail. §3 addresses the mismatch between the logical status of contradictory negation as a propositional operator and the diverse realizations of contradictory negation within natural language. §4 deals with several challenges to LNC within Western philosophy, including the paradoxes, and the relation between systems with truth-value gaps (violating LEM) and those with truth-value gluts (violating LNC). In §5, the tetralemma of Buddhist logic is discussed within the context of gaps and gluts; it is suggested that apparent violations of LNC in this tradition (and others) can be attributed to either differing viewpoints of evaluation, as foreseen by Aristotle, or to intervening modal and epistemic operators. §6 focuses on the problem of “borderline contradictions”: the range of acceptability judgments for apparently contradictory sentences with vague predicates as surveyed in empirical studies, and the theoretical implications of these studies. Finally, §7 surveys the ways of contradiction and its exploitation in literature and popular culture from Shakespeare to social media.

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• 1. LNC as Indemonstrable
• 2. LEM and LNC
• 3. Contradictory Negation in Term and Propositional Logic
• 4. Gaps and Gluts: LNC and Its Discontents
• 5. LNC and the Buddhist Tetralemma
• 6. Vagueness and Borderline Contradictions
• 7. Contradiction in Everyday Life
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries
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1. LNC as Indemonstrable

The twin foundations of Aristotle's logic are the law of non-contradiction (LNC) (also known as the law of contradiction, LC) and the law of excluded middle (LEM). In Metaphysics Book Γ, LNC—“the most certain of all principles”—is defined as follows:

It is impossible that the same thing can at the same time both belong and not belong to the same object and in the same respect, and all other specifications that might be made, let them be added to meet local objections (1005b19–23).

It will be noted that this statement of the LNC is an explicitly modal claim about the incompatibility of opposed properties applying to the same object (with the appropriate provisos). Since Łukasiewicz (1910), this ontological version of the principle has been recognized as distinct from, and for Aristotle arguably prior to, the logical formulation (“The opinion that opposite assertions are not simultaneously true is the firmest of all”—Met. 1011b13–14) and the psychologicalformulation (“It is impossible for anyone to believe that the same thing is and is not, as some consider Heraclitus said”—Met. 1005b23–25) offered elsewhere in Book Γ; we return to Heraclitus below. Wedin (2004a), who argues for the primacy of the ontological version (see also Meyer 2008, Other Internet Resources), formalizes it as ¬◊(∃x)(Fx ∧ ¬Fx). These three formulations of LNC differ in important respects, in particular as to whether the law is explicitly modal in character, whether it applies to propositions or to properties and objects, and whether it requires the invocation of a metalinguistic truth predicate. (See also the entry Aristotle on non-contradiction.)

For Aristotle, the status of LNC as a first, indemonstrable principle is obvious. Those who mulishly demand a proof of LNC clearly “lack education”: since “a demonstration of everything is impossible”, resulting in infinite regress. At least some principles must be taken as primitiveaxiomata rather than derived from other propositions—and what principle more merits this status than LNC? (1006a6–12). In first philosophy, as in mathematics, an axiom is both indemonstrable and indispensable; without LNC, Aristotle argues, “a is F” and “a is not F” are indistinguishable and no argumentation is possible. While Sophists and “even many physicists” may claim that it ispossible for the same thing to be and not to be at the same time and in the same respect, such a position self-destructs “if only our opponent says something”, since as soon as he opens his mouth to make an assertion, any assertion, he must accept LNC. But what if he does not open his mouth? Against such an individual “it is ridiculous to seek an argument” for he is no more than a vegetable (1006a1–15).

The celebrated Arab commentator Avicenna (ibn Sīnā, 980–1037) confronts the LNC skeptic with a more severe outcome than Aristotle's vegetable reduction: “As for the obstinate, he must be plunged into fire, since fire and non-fire are identical. Let him be beaten, since suffering and not suffering are the same. Let him be deprived of food and drink, since eating and drinking are identical to abstaining” (Metaphysics I.8, 53.13–15).

The role of LNC as the basic, indemonstrable “first principle” is affirmed by Leibniz, for whom LNC is taken as interdefinable with the Law of Identity that states that everything is identical to itself: “Nothing should be taken as first principles but experiences and the axiom of identity or (what is the same thing) contradiction, which is primitive, since otherwise there would be no difference between truth and falsehood, and all investigation would cease at once, if to say yes or no were a matter of indifference” (Leibniz 1696/Langley 1916: 13–14). For Leibniz, everybody—even “barbarians”—must tacitly assume LNC as part of innate knowledge implicitly called upon at every moment, thus demonstrating the insufficiency of Locke's empiricism (ibid., 77).[1]

In accounting for the incompatibility of truth and falsity, LNC lies at the heart of Aristotle's theory of opposition, governing both contradictories and contraries. (See traditional square of opposition.) Contradictory opposites (“She is sitting”/“She is not sitting”) are mutually exhaustive as well as mutually inconsistent; one member of the pair must be true and the other false, assuming with Aristotle that singular statements with vacuous subjects are always false. As it was put by the medievals, contradictory opposites divide the true and the false between them; for Aristotle, this is the primary form of opposition.[2] Contrary opposites (“He is happy”/“He is sad”) are mutually inconsistent but not necessarily exhaustive; they may be simultaneously false, though not simultaneously true. LNC applies to both forms of opposition in that neither contradictories nor contraries may belong to the same object at the same time and in the same respect (Metaphysics1011b17–19). What distinguishes the two forms of opposition is a second indemonstrable principle, the law of excluded middle (LEM): “Of any one subject, one thing must be either asserted or denied” (Metaphysics 1011b24). Both laws pertain to contradictories, as in a paired affirmation (“Sis P”) and denial (“S isn't P”): the negation is true whenever the affirmation is false, and the affirmation is true when the negation is false. Thus, a corresponding affirmation and negation cannot both be true, by LNC, but neither can they both be false, by LEM. But while LNC applies both to contradictory and contrary oppositions, LEM holds only for contradictories: “Nothing can exist between two contradictories, but something may exist between contraries” (Metaphysics1055b2): a dog cannot be both black and white, but it may be neither.

As Aristotle explains in the Categories, the opposition between contradictories—“statements opposed to each other as affirmation and negation”—is defined in two ways. First, unlike contrariety, contradiction is restricted to statements or propositions; terms are never related as contradictories. Second, “in this case, and in this case only, it is necessary for the one to be true and the other false” (13b2–3).

Opposition between terms cannot be contradictory in nature, both because only statements (subject-predicate combinations) can be true or false (Categories 13b3–12) and because any two terms may simultaneously fail to apply to a given subject.[3] But two statements may be members of either a contradictory or a contrary opposition. Such statements may be simultaneously false, although (as with contradictories) they may not be simultaneously true. The most striking aspect of the exposition for a modern reader lies in Aristotle's selection of illustrative material. Rather than choosing an uncontroversial example involving mediate contraries, those allowing an unexcluded middle (e.g. “This dog is white”/“This dog is black”; “Socrates is good”/“Socrates is bad”), Aristotle offers a pair of sentences containing immediate contraries, “Socrates is sick”/“Socrates is well”. These propositions may both be false, even though every person is either ill or well: “For if Socrates exists, one will be true and the other false, but if he does not exist, both will be false; for neither ‘Socrates is sick’ nor ‘Socrates is well’ will be true, if Socrates does not exist at all” (13b17–19). But given a corresponding affirmation and negation, one will always be true and the other false; the negation “Socrates is not sick” is true whether the snub-nosed philosopher is healthy or non-existent: “for if he does not exist, ‘he is sick’ is false but ‘he is not sick’ true” (13b26–35).

Members of a canonical pair of contradictories are formally identical except for the negative particle:

An affirmation is a statement affirming something of something, a negation is a statement denying something of something…It is clear that for every affirmation there is an opposite negation, and for every negation there is an opposite affirmation…Let us call an affirmation and a negation which are opposite a contradiction (De Interpretatione 17a25–35).

But this criterion, satisfied simply enough in the case of singular expressions, must be recast in the case of quantified expressions, both those which “signify universally” (“every cat”, “no cat”) and those which do not (“some cat”, “not every cat”).

For such cases, Aristotle shifts from a formal to a semantically based criterion of opposition (17b16–25). Members of an A/O pair (“Every man is white”/“Not every man is white”) or I/E pair (“Some man is white”/“No man is white”) are contradictories because in any state of affairs one member of each pair must be true and the other false. Members of an A/E pair—“Every man is just”/“No man is just”—constitute contraries, since these cannot both be true simultaneously but can both be false. The contradictories of these contraries (“Not every man is just”/“Some man is just”) can be simultaneously true with reference to the same subject (17b23–25). This last opposition of I and O statements, later to be dubbed subcontraries because they appear below the contraries on the traditional square, is a peculiar opposition indeed; Aristotle elsewhere (Prior Analytics 63b21–30) sees I and O as “only verbally opposed”, given the consistency of the Istatement, e.g. “Some Greeks are bald”, with the corresponding O statement, “Some Greeks aren't bald” (or “Not all Greeks are bald”, which doesn't necessarily amount to the same thing, given existential import; see traditional square of opposition).

The same relations obtain for modal propositions, for propositions involving binary connectives like “and” and “or”, for quantificational adverbs, and for a range of other operators that can be mapped onto the square in analogous ways utilizing the same notions of contradictory and contrary opposition and with unilateral entailment definable on duals (see Horn 1989). Thus for example we have the modal square below, based on De Interpretatione 21b10ff. and Prior Analytics 32a18–28, where the box and diamond symbols denote necessity and possibility, respectively. As with universal affirmatives and universal negatives, necessity and impossibility constitute contraries: “A priest must marry” and “A priest can’t marry” can both be (and, on the Episcopalian reading, are) false but cannot both be true. “A priest can marry” and “A priest can (if he wants) not marry” are subcontraries; these can be simultaneously true but not simultaneously false. And necessity, as in “A priest must marry”, unilaterally entails its possibility dual counterpart, “A priest may marry”.

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(1) Modal Square

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#Aristotle Returns

 

 by Tim Crane  2018

Neo-Aristotelian Perspectives on Contemporary Science 

Raphael’s School of Athens depicts Aristotle and Plato at the center of a group of ancient Greek philosophers modeled on Raphael’s contemporaries. Plato’s finger points upward, while Aristotle’s hand is held at waist height, stretched out toward the ground. The image captures the major philosophical difference between the two great thinkers of antiquity: While Plato thought that real things (what he called the “forms”) lie outside our experience, Aristotle believed that real things (which he called “substances”) are in the everyday world around us. This is why so much of Aristotle’s work covers topics which would now be the subject of empirical science (animal biology, the weather, and so on). With his commitment to the reality of the ordinary things we experience and his tireless need to classify and taxonomize, Aristotle was in some ways the first systematic scientist of the Western world.

 

Aristotle’s view that substances are the fundamental realities was put forward in his difficult and influential work known as the Metaphysics. We owe the term “metaphysics” itself to Aristotle, though in a rather indirect way. When his works were collected in the edition made by ­Andronicus of Rhodes in the first century b.c., the writings dealing with substance and causation were placed after his theory of nature, which appeared in the book known as the Physics (from the Greek word for nature, phusis). Hence, the work was described as “what comes after the Physics,” from which we derive our word metaphysics. Yet this word has something of a philosophical justification, too, for while Aristotle’s Physics deals with questions about time and change, the Metaphysics goes further (“after physics,” as it were) and asks about being as such. 

What does it mean for anything to be? 

Aristotle approaches this question by describing the nature of the most fundamental beings, those he called “primary substances.” His paradigm of a substance is an organism: an individual horse or human being. It is crucial, then, that Aristotle’s word “substance” does not mean “stuff” (as in “tar is a thick, black, viscous substance”). It means, rather, a ­fundamental being. 

Primary substances are characterized by their essences. All beings can be classified in terms of genera and species: Thus, human beings are of the genus animal, and the species rational. It is of the essence of human beings in general, then, to be rational animals. But what is it to be a primary or individual substance—say, an individual human being? ­Aristotle argued that each individual substance was a kind of compound, made of matter organized in a certain way. The way in which something is organized he called its form. This is known as the “hylomorphic” view of substance, from the Greek hyle (matter) and morphe (form). 

 

The form of an individual substance is the essence of the kind of substance, exemplified in an individual member of that kind. This is the crucial difference between the Aristotelian and Platonic conceptions of form. For Plato, the ordinary things we see around us are in some way imperfect versions of the real forms which lie outside ­experience. The forms are not in the objects themselves; if anything, the objects approximate to their forms. Individual horses approximate to, or participate in, or imperfectly resemble the “form of the horse.” 

This is the first of your three free articles for the month.

For Aristotle, by contrast, the form of a substance is in the substance itself (in traditional terminology, it is immanent, rather than transcendent). Moreover, the form explains the characteristic activity of a substance, as being in accordance with what it is ­ultimately for: The form is the principle that governs the activity of a substance, that which explains why a substance does what it does. The goal of a human being, for example, is to live a life of rational activity; something having the essence of a rational animal will strive toward this goal. Thus Aristotle’s theory of substance is sometimes called a “teleological” theory: It describes a substance in terms of its characteristic goal (telos in Greek). 

It is a commonplace of the history of science and philosophy that this Aristotelian philosophy—which dominated the academic (or scholastic) philosophy of the Middle Ages in the monasteries and universities of Europe—was overthrown by the Scientific Revolution of the sixteenth and seventeenth centuries, a revolution embodied in the works of Galileo, Francis Bacon, and René Descartes. Scholastics had thought of everything in the world as having purposes and goals. The new philosophers of the Scientific Revolution conceived the world instead in terms of mathematically measurable mechanisms. Galileo famously spoke of “this grand book the universe, which . . . is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures, without which it is humanly impossible to understand a single word of it.” The new mathematical or “mechanical” philosophy scorned the Aristotelian philosophy of the scholastic period. Francis Bacon said that Aristotle “made his natural philosophy a mere bond-servant to his logic, thereby rendering it contentious and well nigh useless.” In other words, his philosophy of nature was a priori (prior to experience) and based purely on the concepts of things ­derived from logical classification. Despite misrepresenting Aristotle’s own dedication to empirical inquiry, this kind of criticism stuck. In the wider culture, the rejection of scholasticism was immortalized by the remark of the doctor in Molière’s play The Hypochondriac, that opium puts us to sleep because it has a “dormitive virtue” (virtus dormitiva).

This customary story of the Scien­tific Revolution and its rejection of ­Aristotelian scholasticism is incomplete and inaccurate in many ways. Not all great philosophers of the period rejected Aristotelian thinking in its entirety. Gottfried Wilhelm Leibniz (1646–1716) criticized scholastic philosophers who “believed they could explain the properties of bodies by referring to forms and qualities, without taking the trouble to find out how they worked: as if we were happy to say that a clock has a time-indicative quality deriving from its form, without considering what all that amounted to.” But despite this criticism, ­Leibniz said that Aristotelian substantial forms “are not so far from the truth, nor so ridiculous, as the common run of our new philosophers suppose.” Substance and form could be used to explain the ultimate nature of things, Leibniz believed, even if they are not needed to explain ordinary, everyday things, like the time-keeping properties of clocks. It is also important to remember that within the Catholic intellectual tradition, Aristotelian ideas continued to be taught in universities and seminaries for centuries after the Scientific Revolution. 

Nonetheless, Aristotelian metaphysics was largely absent from what might be called (with apologies to the seminaries) mainstream philosophy from Descartes to the present day. Metaphysics took various forms between then and now, but almost none of it involved Aristotelian notions of telos, form, or substance. The history of philosophy in the twentieth century is instructive. In English-speaking (“analytic”) philosophy, the attitude to metaphysics—that is, to abstract, non-scientific, or non-­empirical speculation about the nature of reality—was dominated by the critique of the logical positivists of the pre–World War II Vienna Circle. These philosophers proposed that verifiability by empirical science was the measure of meaningfulness for claims about the world. Metaphysical claims—­including but not limited to traditional Aristotelian claims about substance, essence, form, and so on—would be examples of the ­unverifiable. Metaphysics was dismissed as outdated pseudoscience.

Some logical positivists traced their inspiration from the philosophy of David Hume (1711–1776), who had proposed a division of claims into those that are “matters of fact” and those that involve only “relations of ideas.” Ordinary empirical claims are in the first category, while mathematics and logic are in the second. If some inquiry—Hume famously took “divinity or school metaphysics” as an example—fits into neither category, then his advice was to “commit it then to the flames: for it can contain nothing but sophistry and illusion.” This sums up the logical positivists’ approach to metaphysics (and theology, for that matter).

Nonetheless, Hume had a metaphysics of his own—a rather austere one, but a metaphysics nonetheless. His metaphysics denied that there was any necessity in reality at all: Things happen as they do not because of any necessity or essence or ultimate reason, but because they just do. The world is a regular place; things happen in generally unsurprising ways; but this is not an indication of any deeper necessity in nature. Causation—what Hume called “the cement of the Universe”—is just a matter of the “constant conjunction” of things of similar kinds: Smoke always follows fire, and so on. Defenders of Hume’s metaphysics called the summaries of these regularities “laws” and interpreted scientific laws as generalizations of this kind. With the demise of logical positivism in the 1950s and ’60s, Humean theories of cause and law became the foundations of metaphysics in analytic philosophy. Indeed, if you had left the world of philosophy in the 1970s, you might have thought that Humean, empiricist, science-based metaphysics was the only metaphysics worth taking seriously. 

Things did not stay like this, however. Aristotelian metaphysics started to return, and the volume under review is one of many books that have come out in recent years defending Aristotelian views of causation, substance, attribute, and even essence and form. How did this change come about? And how can Aristotelian ­metaphysics—which, if the standard history is to be believed, was rejected on the basis of discoveries of modern ­science—be part of a serious, scientific worldview? How can serious thinkers propose “neo-Aristotelian perspectives” on contemporary ­science? John ­Haldane, in the ­preface to Neo-Aristotelian Perspectives, poses the challenge by asking why this project isn’t like “astrological perspectives on astronomy.”

The first factor in the rebirth of Aristotelianism came from an unexpected place: formal logic, in particular the logical theories developed in the 1960s and ’70s by Saul Kripke, one of the great philosophers of the late twentieth century. A teenage prodigy, Kripke developed a way to understand the logic of possibility and necessity (so-called “modal logics”), which led to certain natural metaphysical interpretations. Philosophers had talked for some time in terms of necessary truth as truth in all possible worlds. Kripke introduced a precise way of formulating this idea, and pursued its interpretation into a metaphysics of essence and necessity. One central idea was that if we want to make sense when we say that something is necessarily such and such (for example, that a person is necessarily human), we should think of this as a feature that it has in all possible worlds in which it exists. But if this is the right way to think, then we must also be able to identify the same individual in these different possible worlds. And in order to do this, there must be something about it which makes it the individual it is in each of the worlds in which it exists. And what is this but its essence? 

Kripke extended the idea of essence beyond individuals to kinds of things, such as gold and water. We can imagine that gold might have a slightly different color, for example, but what makes it the case that it is gold we are imagining, rather than something that is rather like gold in certain respects? Kripke argued that to identify gold in different possible situations requires that gold have an essence. He proposed that the essence of an element is its atomic number, so gold is essentially the element with the atomic number 79. The details do not matter here; what was important was the idea that the world has a natural order, an order which is neither imposed by our interpretation nor just the order of Humean laws of nature. Human beings discovered that water is H₂O; we did not invent this fact. What we were discovering, Kripke and his followers argued, is not just a law or regularity: It is rather the essence of the natural kind water. These ideas are clearly Aristotelian in inspiration. 

Essences are, of course, anathema to Humean metaphysics and to the post-positivist philosophy of W. V. Quine and his followers (Quine himself said that Aristotle’s distinction between essence and accident is “surely indefensible”). But armed with Kripke’s logical and metaphysical framework, people could defend essence against the Quinean critique. More recently, self-identified Aristotelians such as Kit Fine have argued that the natural order of the world requires that certain things are more fundamental than others, and that fundamental things stand in a relation to non-fundamental things that can be called “grounding.” For example, the United Nations genuinely exists; it is an entity, but no one would claim it is a fundamental entity. The United Nations is made up of entities, nations, which perhaps have more of a claim to be real, and these things themselves are made up of entities, human beings, which an Aristotelian would call substances. It is asserted that the less fundamental entities can be grounded in the being and activity of more fundamental entities. (In the volume under review, Robert Koons discusses grounding in the context of an Aristotelian philosophy of physics.)

The second area of philosophy in which Aristotelian ideas has returned is the philosophy of causation (or causality): the study of cause and effect. According to Hume’s influential conception of causation, “all events seem to be entirely loose and separate.” There is no necessary connection between distinct existences. Yet Humeans always had trouble with causal relations that were a result of so-called “dispositional” properties: solubility, fragility, and so on. These are properties that are described in terms of the effects that their possession is “disposed” to bring about—dissolving, breaking, etc. How can all events be “loose and separate” if some events (such as dissolving) seem to be essentially or metaphysically connected to their causes (solubility)?

Humeans have responded by arguing that such dispositionality is not a real feature of the world, but only an artifact of our description of it. What causes dissolving is the interaction between certain events (being put in water, say) and the “structural” or “categorical” properties of things (molecular structure, and so on). And these interactions are covered by laws of nature, as per the usual Humean theory of causation. Defenders of dispositions push back against this: How do we actually specify these supposed “structural” properties? Isn’t structure partly characterized in dispositional terms (for example, in terms of the ability to resist pressure at various points)? 

This line of thought can be supported by looking closely at what the laws of nature actually say. Many laws of nature characterize the properties of things in terms of the effects that an object’s having these properties lead to. For example, Newton’s second law of motion defines the force exerted on a body as the product of its mass and its acceleration (force equals mass times acceleration or f=ma). Defenders of the reality of dispositions argue that this law can be understood as a definition of what, say, an object’s mass—one of its properties—really is: It is a disposition to accelerate under a given force, in accord with that equation. So rather than just describing a mere regularity between “loose and separate” existences, as the Humeans say, laws like Newton’s should be understood as defining the nature of properties in terms of their characteristic effects—that is, in dispositional terms.

This kind of challenge to the Humean view led to the development of sophisticated theories of properties understood as “causal powers.” This terminology was introduced in an influential book by Rom Harré and E. H. Madden in 1975, and the basic idea has been developed more recently in various ways by George Molnar (Powers, 2003), Stephen Mumford and Rani Lill Anjum (Getting Causes from Powers, 2011), and Anna ­Marmodoro (The Metaphysics of Powers, 2010). Marmodoro is one of the recent philosophers who has brought out most explicitly the Aristotelian roots of the powers-­dispositions view of causation, linking it to ­Aristotle’s famous discussions of potentiality and actuality. Her work is well represented in the volume under review by an essay (cowritten with Christopher Austin) linking the notion of potentiality with the concept of the unity of an organism. In an interesting and complementary discussion, Janice Chik Breidenbach’s essay defends the Aristotelian view that substances themselves can be causes (it was the horse that knocked down the gate, not just some event involving, for instance, the horse’s legs). And Humeanism in metaphysics as a whole is discussed in William Simpson’s essay on why dispositions cannot be fully or properly accommodated within a Humean framework. 

 

A third important factor in the revival of Aristotelian metaphysics comes from the development of the philosophy of science itself. Again, the crucial immediate historical precursors were the logical positivists and those influenced by them. The logical positivists had seen scientific theories as aiming at the statement of laws of nature which were as general and exceptionless as possible. The paradigm was physics: Laws such as Newton’s aim to state the most general truths about how the universe behaves. According to the positivists, the fact that Newton’s laws are not actually true should not be blamed on their claim to generality. Rather, these philosophers looked to replace these laws with laws that were equally general. This idea—­science aims at laws, and laws should be regarded as statements that are as general and exceptionless as possible—came under critical scrutiny in work from the 1970s and ’80s. This work emphasized the partial, local nature of scientific theory, the importance of modeling in science, and the unrealistic character of the positivists’ description of science as a list of statements of universal laws. An influential figure here is Nancy Cartwright, who gave an alternative description of science in terms of the measurement of “Nature’s capacities.” Rather than being something of which one single scientific story could be told—in a “theory of everything,” as it were—the world is, in Cartwright’s image, irreducibly “dappled.” It is made up of a plurality of different kinds of things, about which there is no one fundamental account, only separate accounts for separate kinds of things and their various capacities. This is clearly an Aristotelian picture, as Cartwright herself acknowledges, with its emphasis on capacities (a notion closely related to that of a power or disposition) and on things being of different kinds. In Neo-­Aristotelian ­Perspectives, Xavi ­Lanao and ­Nicholas Teh apply some of ­Cartwright’s ideas to argue that even classical mechanics does not conform to the “fundamentalist” picture of science criticized by ­Cartwright.

To these factors must be added a fourth factor, which is often not explicitly credited as an influence on the present rebirth of Aristotelianism: the influence of Catholic philosophy and its own resilient metaphysics. It will not be news to readers of this journal that Catholic seminaries and universities continued to teach Thomistic philosophy (itself a form of Aristotelianism), and leading Thomist (or Thomism-inspired) philosophers such as Bernard Lonergan have had a wide influence, albeit one which rarely made contact with mainstream metaphysics in the twentieth century. Yet the revolt against the dominant Humean metaphysics in recent decades has led to more dialogue (and even collaboration) between Catholic and what I am calling mainstream philosophy. The present book is a good example. Many of the contributors are Roman Catholics—some known independently for their work in the philosophy of religion—and some teach at ­Catholic universities in the United States. Their Catholicism plays little direct role in their actual philosophical contributions to this volume, but it provides the intellectual framework within which many of these thinkers work. 

 

One way in which Thomistic metaphysics influences the rest of philosophy, for example, is in the question of the soul and its relation to the body. It is easy to see why this should be such an important question for Catholics. The traditional Thomistic view, drawn from Aristotle, is hylomorphic: The soul is the form of the body’s matter. For Aristotle, any substance (like a human being) is a compound of form and matter; the matter cannot exist without the form, and the form cannot exist without the matter. In the current volume, ­William Jaworski gives a clear interpretation of how this view of the human being might “leave it unmysterious how thought, feeling and perception can exist in the natural world.” Another important area where contemporary hylomorphists apply their ideas is the philosophy of biology, exemplified here in fine essays by David Oderberg and Daniel De Haan. 

I hope that these largely historical considerations begin to clarify why Aristotelian philosophy (in particular, metaphysics) has returned. But I hope they also show how Aristotelian metaphysics can be scientifically defensible. Remember that the worry was that the Scientific Revolution of the sixteenth and seventeenth centuries saw Aristotelian philosophy as unscientific—in a sense, the traditional view is that science as a systematic attempt at genuine explanation only began with the rejection of Aristotelianism and scholasticism.

But it should be noted that these are philosophical claims, not scientific ones. Like the positivists’ claim that only what can be verified can be genuinely meaningful, or the “scientistic” claim that science can explain everything, these are not claims made within scientific theories themselves. Indeed, metaphysical claims—about substance, cause, change, potentiality, etc.—are very rarely settled by the content of scientific theories. If you are interested in how change is possible, you will not look to physics or chemistry. Physicists and chemists presuppose that change is possible and then go on to talk about the specific nature of specific changes. Similarly, physics and chemistry do not settle the question of whether the physical world is all there is; that question must be left to metaphysics. 

 

Although science is not itself metaphysics, metaphysics of science is ­unavoidable. Once we start theorizing at a certain level of generality, we cannot escape metaphysical commitments. For example, if we ask what kind of entities physical theories are committed to, we may have to answer in terms of traditional categories such as substance, property, object, ­process—or specify some new categories. This does not mean that physicists must be metaphysicians, only that if they enter into metaphysical speculation, they should acknowledge that others have been there before them, and that the questions are not easy. For example, in attempting to argue that a causal connection exists ­between some phenomena, it is common to find scientists saying “correlation is not causation”—and they are right. But what is causation, then, and how does it differ from mere correlation? Answering this requires metaphysics. Those philosophers and scientists who dismiss ­metaphysics, often casually and without much argument, have to demonstrate how they can do this without doing ­metaphysics themselves. I predict that they will not be able to do this. Even the logical positivists had metaphysical assumptions.

The lesson of this is that seventeenth-century science did not prove that Aristotelian metaphysics failed—this was simply a claim made by philosophers. Whether science requires specific metaphysical assumptions or not is itself a metaphysical debate, which requires knowledge of science but is not settled by it. Aristotelian metaphysical categories—substance, form, capacity, essence—can be intelligibly applied to the findings of science. Or, at least, there are no ­scientific arguments against this. This is why Neo-Aristotelian Perspectives on Contemporary Science is not analogous to neo-astrological perspectives on contemporary astronomy.

 

Tim Crane is professor of philosophy at the Central European University