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Aristotle: Logical Methods

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The greatest and most influential of Plato's students was Aristotle, who established his own school at Athens. Although his writing career probably began with the production of quasi-Platonic dialogues, none of them have survived. Instead, our knowledge of Aristotle's doctrines must be derived from highly-condensed, elliptical works that may have been lecture notes from his teaching at the Lyceum. Although not intended for publication, these texts reveal a brilliant mind at work on many diverse topics.

Philosophically, the works of Aristotle reflect his gradual departure from the teachings of Plato and his adoption of a new approach. Unlike Plato, who delighted in abstract thought about a supra-sensible realm of forms, Aristotle was intensely concrete and practical, relying heavily upon sensory observation as a starting-point for philosophical reflection. Interested in every area of human knowledge about the world, Aristotle aimed to unify all of them in a coherent system of thought by developing a common methodology that would serve equally well as the procedure for learning about any discipline.

For Aristotle, then, logic is the instrument (the "organon") by means of which we come to know anything. He proposed as formal rules for correct reasoning the basic principles of the categorical logic that was universally accepted by Western philosophers until the nineteenth century. This system of thought regards assertions of the subject-predicate form as the primary expressions of truth, in which features or properties are shown to inhere in individual substances. In every discipline of human knowledge,then, we seek to establish the things of some sort have features of a certain kind.

Aristotle further supposed that this logical scheme accurately represents the true nature of reality. Thought, language, and reality are all isomorphic, so careful consideration of what we say can help us to understand the way things really are. Beginning with simple descriptions of particular things, we can eventually assemble our information in order to achieve a comprehensive view of the world.

Applying the Categories

The initial book in Aristotle's collected logical works is the Categories, an analysis of predication generally. It begins with a distinction among three ways in which the meaning of different uses of a predicate may be related to each other: homonymy, synonymy, and paronymy (in some translations, "equivocal," "univocal," and "derivative"). Homonymous uses of a predicate have entirely different explanations, as in "With all that money, she's really loaded," and "After all she had to drink, she's really loaded." Synonymous uses have exactly the same account, as in "Cows are mammals," and "Dolphins are mammals." Paronymous attributions have distinct but related senses, as in "He is healthy," and "His complexion is healthy." (Categories 1) It is important in every case to understand how this use of a predicate compares with its other uses.

So long as we are clear about the sort of use we are making in each instance, Aristotle proposed that we develop descriptions of individual things that attribute to each predicates (or categories) of ten different sorts. Substance is the most crucial among these ten, since it describes the thing in terms of what it most truly is. For Aristotle, primary substance is just the individual thing itself, which cannot be predicated of anything else. But secondary substances are predicable, since they include the species and genera to which the individual thing belongs. Thus, the attribution of substance in this secondary sense establishes the essence of each particular thing.

The other nine categories—quantity, quality, relative, where, when, being in a position, having, acting on, and being affected by—describe the features which distinguish this individual substance from others of the same kind; they admit of degrees and their contraries may belong to the same thing. (Categories 4) Used in combination, the ten kinds of predicate can provide a comprehensive account of what any individual thing is. Thus, for example: Chloë is a dog who weighs forty pounds, is reddish-brown, and was one of a litter of seven. She is in my apartment at 7:44 a.m. on June 3, 1997, lying on the sofa, wearing her blue collar, barking at a squirrel, and being petted. Aristotle supposed that anything that is true of any individual substance could, in principle, be said about it in one of these ten ways.

The Nature of Truth

Another of Aristotle's logical works, On Interpretation, considers the use of predicates in combination with subjects to form propositions or assertions, each of which is either true or false. We usually determine the truth of a proposition by reference to our experience of the reality it conveys, but Aristotle recognized that special difficulties arise in certain circumstances.

Although we grant (and can often even discover) the truth or falsity of propositions about past and present events, propositions about the future seem problematic. If a proposition about tomorrow is true (or false) today, then the future event it describes will happen (or not happen) necessarily; but if such a proposition is neither true nor false, then there is no future at all. Aristotle's solution was to maintain that the disjunction is necessarily true today even though neither of its disjuncts is. Thus, it is necessary that either tomorrow's event will occur or it will not, but it is neither necessary that it will occur nor necessary that it will not occur. (On Interpretation 9)

Aristotle's treatment of this specific problem, like his more general attempt to sort out the nature of the relationship between necessity and contingency in On Interpretation 12-13, is complicated by the assumption that the structure of logic models the nature of reality. He must try to explain not just the way we speak, but the way the world therefore must be.

Demonstrative Science

Finally, in the Prior Analytics and Posterior Analytics, Aristotle offered a detailed account of the demonstrative reasoning required to substantiate theoretical knowledge. Using mathematics as a model, Aristotle presumed that all such knowledge must be derived from what is already known. Thus, the process of reasoning by syllogism employs a formal definition of validity that permits the deduction of new truths from established principles. The goal is to provide an account of why things happen the way they do, based solely upon what we already know.

In order to achieve genuine necessity, this demonstrative science must be focussed on the essences rather than the accidents of things, on what is "true of any case as such," rather than on what happens to be "true of each case in fact." It's not enough to know that it rained today; we must be able to figure out the general meteorological conditions under which rain is inevitable. When we reason from necessary universal and affirmative propositions about the essential features of things while assuming as little as possible, the resulting body of knowledge will truly deserve the name of science.

The Four Causes

Applying the principles developed in his logical treatises, Aristotle offered a general account of the operation of individual substances in the natural world. He drew a significant distinction between things of two sorts: those that move only when moved by something else and those that are capable of moving themselves. In separate treatises, Aristotle not only proposed a proper description of things of each sort but also attempted to explain why they function as they do.

Aristotle considered bodies and their externally-produced movement in the Physics. Three crucial distinctions determine the shape of this discussion of physical science. First, he granted from the outset that, because of the difference in their origins, we may need to offer different accounts for the functions of natural things and those of artifacts. Second, he insisted that we clearly distinguish between the basic material and the form which jointly constitute the nature of any individual thing. Finally, Aristotle emphasized the difference between things as they are and things considered in light of their ends or purposes.

Armed with these distinctions, Aristotle proposed in Physics II, 3 that we employ four very different kinds of explanatory principle {Gk. αιτιον [aition]} to the question of why a thing is, the four causes:

The material cause is the basic stuff out of which the thing is made. The material cause of a house, for example, would include the wood, metal, glass, and other building materials used in its construction. All of these things belong in an explanation of the house because it could not exist unless they were present in its composition.

The formal cause {Gk. ειδος [eidos]} is the pattern or essence in conformity with which these materials are assembled. Thus, the formal cause of our exemplary house would be the sort of thing that is represented on a blueprint of its design. This, too, is part of the explanation of the house, since its materials would be only a pile of rubble (or a different house) if they were not put together in this way.

The efficient cause is the agent or force immediately responsible for bringing this matter and that form together in the production of the thing. Thus, the efficient cause of the house would include the carpenters, masons, plumbers, and other workers who used these materials to build the house in accordance with the blueprint for its construction. Clearly the house would not be what it is without their contribution.

Lastly, the final cause {Gk. τελος [télos]} is the end or purpose for which a thing exists, so the final cause of our house would be to provide shelter for human beings. This is part of the explanation of the house's existence because it would never have been built unless someone needed it as a place to live.

Causes of all four sorts are necessary elements in any adequate account of the existence and nature of the thing, Aristotle believed, since the absence or modification of any one of them would result it the existence of a thing of some different sort. Moreover, an explanation that includes all four causes completely captures the significance and reality of the thing itself.

The Appearance of Chance

Notice that the four causes apply more appropriately to artifacts than to natural objects. The rise of modern science resulted directly from a rejection of the Aristotelean notion of final causes in particular. Still, the scheme works so well for artifacts that we often find ourselves attributing some purpose even to the apparently pointless events of the natural world.

In many applications the formal, efficient, and final causes tend to be combined in a single being that designs and builds the thing for some specific purpose. Thus, the fundamental differentiation in the Aristotelean world turns out to be between inert matter on the one hand and intelligent agency on the other. As we shall soon see, this provides a natural explanation for the functions of animate natural organisms.

As for things that appear to arise by pure chance, Aristotle argued that since the purposeful origination described by the four causes is the normal order of the world, these instances must either be things that should have had some cause but happen to lack it or (more likely) things that actually do have causes of which we are simply unaware. The craft evident in the manufacture of artifacts, he believed, is evidence for the purposive character of nature, and it shares the same necessity, even though we are sometimes ignorant of its internal operations. (Physics II, 8)

Although I would be hard-pressed to come up with a final cause for the existence of the mosquito that is now biting me, for example, Aristotle supposed that there must ultimately be some explanation for its present existence and activity. Many generations of Western philosophers, especially those concerned with reconciling Christian doctrine with philosophy, would explicitly defend a similar view.

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