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
- The Organon
- Categories
- From Words into Propositions
- Kinds of Propositions
- Square of Opposition
- Laws of Thought
- Existential Assumptions
- Form versus Content
- The Syllogism
- Inductive Syllogism
- Deduction versus Induction
- Science
- Non-Discursive Reasoning
- Rhetoric
- Fallacies
- Moral Reasoning
- References and Further Reading
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.
