The Aristotelian Theory of Knowledge
"Ancient" philosophy is often contrasted with "Modern" philosophy (i.e. philosophy from the Enlightenment through late 19th century) by saying that the latter focused on knowing whereas the former was concerned with being. This would misleadingly suggest that epistemology took a backseat to metaphysics in ancient philosophy and that the engagement with epistemology is a distinctly modern project. The philosophy of Aristotle presents a vivid counterexample to this cliche'. To be sure, modern philosophers did pose epistemological questions in a way which raised a partially different set of questions from those with which Aristotle was concerned, but concern with the differences between ancient and modern approaches to knowledge should not blind us to the fact that in matters epistemological both ancients and moderns shared a wide range of presuppositions about the nature of knowledge, and this shared core was, in effect, the contribution of Aristotle.
A second misunderstanding about Aristotle arises from the common pedagogical approach of pitting Aristotle against his mentor, Plato. Against Plato's other-worldliness, Aristotle is this worldly; against Plato's extreme rationalism, Aristotle is an empiricist. Plato stands for the eternal universal embodied in mathematics; Aristotle for the contingent perishable particular of biology. For example in the Renaissance, Raphael painted "The School of Athens" with Plato pointing heavenward and Aristotle pointing earthward. To be sure, Aristotle raises many criticisms against Plato's conception of "Forms," but when it comes to their theories of knowledge they agreed far more than they disagreed. The cliche's in which Aristotle is opposed to Plato should be resisted.
The heart of what it was that both Ancients and moderns shared lies in Aristotle's contributions to logic, collected under the title of the Organon. Not only did Aristotle basically "invent" the subject of logic as a discipline, give it its basic terminology, and set its agenda of problems; but also he managed to say almost everything worth saying about the subject that was said prior to the last hundred years. By the time of Descartes, two thousand years after Aristotle lived, it had become quite fashionable to scorn Aristotle, but Descartes' ideal of knowledge as like a vast geometry, a deductive system of propositions proved to follow with certainty from indubitable axioms was not his creation, but an ideal already envisioned in Aristotle's Organon, and is implicit in Plato's "Divided Line" conception of "dianoia.".
Furthermore Descartes and all his fellow modern philosophers right down through Kant take it absolutely for granted that knowledge is expressed inwhat wer then called "judgments" (or as we would say today, in a less "mentalistic" but more "linguistic" vocabulary. "propositions") which have the logical forms that Aristotle had classified and named. Here Aristotle first stated the central notion that knowledge is expressed in "propositions" or "statements" with simple subject predicate form, and that this represents the metaphysical (or "ontological") relation between what he called "beings" (ousia, in Greek), but which has traditionally been translated as "substances," (the subjects of predication) and their "properties" or "attributes" (the predicates attributed to the subject). Indeed, from the point of view of much contemporary (i.e., 20th century) epistemology, which has relinquished the "modern" (i.e. Enlightenment) goal of certainty, Aristotle appears to have understood the contingent nature of knowledge acquisition (i.e. its dependence on an inductive inference from particular observations) in a much broader and more reasonable way than the rather narrow views typical of modern philosophers.
However, the study of Aristotelian logic is not our goal here, suffice it to say that Aristotle recognized with perfect clarity that the logician's syllogistic demonstration of his conclusion can provide knowledge only if it is deduced validly from true premisses. Those premises could be known to be true if they were deduced from "higher" premises which were known to be true, but how can these premises in their turn, be known to be true? This question suggests a "regress" that must end in some first premiss not known by deduction from some yet higher premiss. Aristotle calls such foundational premises "basic truths." Thus he concludes there can be knowledge by demonstration (deduction) only if there is knowledge of such basic truths by some other means than deduction. Like Plato, Aristotle concludes that this knowledge takes as its object the universal form or essence inherent in the particular primary substance.
Aristotle agrees with Plato that knowledge is of what is true and that this truth must be justified in a way which shows that it must be true, it is necessarily true. Since physical particulars, the "beings" or "substances" of which reality is composed can change, the object of knowledge cannot be the particular, but must be of that which is "universal." When I know "Fido is a dog." I know by sensory observation an individual particular perishable substance, the dog "Fido." But in knowing he is a dog, what I know, the object of my knowledge, is the universal, "Dogness" which is found not only in Fido, but also millions of other substances; it is the "commensurate universal" that which all the particulars exhibiting a form have in common. This is the form, so again Aristotle agrees with Plato that the object of knowledge is the universal form or, as it came to be known, "essence" or "essential nature".
However, the Aristotelian form differs from the Platonic in that it is absolutely in the substance (in Latin, "in rem" as opposed to the Platonic "ante re" signifying Plato's view that the form's existence is independent of the physical particular), so there is nothing "other worldly" about the object of knowledge for Aristotle. Furthermore, also unlike Plato, in Aristotle there is no putting down of sensory perception as a hindrance (almost) to knowledge, in order to elevate the mental. I become acquainted with the form in the substance directly, or "immediately"; I see the dogness in Fido. Thus it is through the senses that we begin to gain knowledge of the form which makes the substance the particular substance it is. But while the process begins with sensory perception, genuine "knowledge" is not delivered simply in the act of perception, but rather is attained only in the "judgment" that what I perceive has this particular form. From the memory records of repeated perceptions, Aristotle tells us, "experience" is born. [Note that this Aristotelian definition of "experience" as requiring the miind's ability to somehow find what is common in the memory's records is altogether different from the way the same word is used in Enlightenment philosophy, which opposes that which we know "by experience" to that which is known by the "mind."] By definition, the judgments of "experience" are of the universal; by experience I know the particular Fido I perceive is a dog, i.e. that Fido has in himself the form of "Dogness" which is not particular to him but universal to all dogs, that which they all share in common and in virtue of which they are dogs and known as dogs; it is the "essential nature" of what-it-is-to-be-a-dog. In a particular sensation I am aware of the sensory particular labeled "Fido," but my knowledge that he is a "dog" is not given by the senses but by the mental act of judging the particular sensations by which I am aware of Fido to be sensations of a dog, a judgment which is made possible only by "experience" of what is common, i.e. universal, in the memory's records of its sensations of many particular dogs. The "organ" that apprehends the universal in experience is not the senses, but nous, i.e., "mind," specifically that "faculty" of the mind which comes to be called "rational intuition" signifying what is "given directly" to the mind.
The judgments which form the foundations of all scientific knowledge, the basic truths, are propositions expressing the nature of the form which is known only through experience. Thus Aristotle seems like a modern empiricist in holding that through experience, we come to know the basic truths of each particular science (and then deduction takes over from there), but those basic truths take as their objects the forms or essences which are apprehended by the mind, not the particular sensations of the senses. The process of knowledge acquisition begins in sensation of the particular, but arrives at its goal only with the mind's grasp of the commensurate universal. Thus while Aristotle's epistemology certainly has greater respect for experience than does Plato's, it is not an empiricism of the sort associated with modern philosophers who sought to make the sensory perceptions themselves the foundations of knowledge.
The division of primary substances follows the pattern of a genus-species hierarchy, in a way already made familiar in Plato's methodology, and very probably "invented" by the historical Socrates. At the bottom are the lowest level species (these are called "infimae species") which are classes of certain "natural kinds" whose members are many primary substances, all sharing a common form or essence. But each lowest level form is what it is because of the higher forms which participate in it, in the fashion of a pyramidal hierarchy already familiar in biology. Fido has in him the form of dogness, but dogness includes the higher essential natures of mammal, animal, living thing, etc. Thus knowledge of the forms of the infimae species leads to knowledge of more general forms, on up, until we arrive at the apprehension of the basic truths about all being, i.e. metaphysics. These most basic of all truths in turn make primary substances what they are; they are the justifiers. All judgements which express those properties which are predicate of a substance essentially are the "basic truths" appropriate to the knowledge of the "science" which concerns substances of that form. While the process of knowledge acquisition (ordo cognescendi) moves from sensation to the mind's grasp of the form, in the order of being (ordo essendi) truths about the highest level forms justify the lower level. Thus Aristotle's whole epistemology turns on his metaphysics of primary substances which are what they are because of the forms or natural essences they embody.
Aristotle's theory of forms is also tied up with his view that we have scientific knowledge of a primary substance only when we know what are usually called its "causes." The Greek word, aitia, which is translated as "causes" is probably better rendered as "that which explains." What this means is that what we know is known only if we are able to explain why our judgment is true, in our epistemological vocabulary, to "justify" it. Thus if the essence of being a dog includes being a quadruped, we are able to explain Fido's four legs by appeal to the form of dogness which is in him. So knowledge of the form or essence is in effect knowledge of the thing's causes, of what explains why it is what it is. In this way Aristotle's theory of knowledge is integrated with his metaphysics.
A full account of the Aristotelian account of the "causes" of a primary substance would require a complete tour through Aristotelian metaphysics, which is beyond present purposes. For epistemological purposes we need only note that the usual listing of the causes as of four different kinds does not conflict with what has been said about the Aristotelian theory of forms or essential natures as the objects of knowledge. Scholars are very far from agreement over Aristotle's view of the "causes", but the usual categorization which came down through history is the formal cause (i.e. the form in the substance), the agent or "efficient" cause (i.e. the being which brought this being into existence, the "creator" from which it comes), the final or teleological cause (i.e. the goal or purpose (telos) towards which it "moves"), and the material cause (i.e. that which "individuates" this particular substance from the other substances which are members of this infimae species).
In the development of his metaphysics on the one hand Aristotle's conception of the forms as "entelechies" ultimately identifies both the agent cause and the final cause with the form in the substance. The material cause as that which is purely particular and exhibits no form whatsoever, totally unformed matter or "prime matter" becomes entirely unknowable.
Thus the conclusion of Aristotle's metaphysics is that -as a necessary and sufficient condition for "knowledge"- we have knowledge of the basic truths of a "science" concerned with the primary substances of a certain natural kind if and only if the mind (from experience of memories provided by sensation) grasps the universal form common to all substances of that natural kind.