The Platonic theory of knowledge is divided into two parts: a quest first to discover whether there are any unchanging objects and to identify and describe them and second to illustrate how they could be known by the use of reason, that is, via the dialectical method. Plato used various literary devices for illustrating his theory; the most famous of these is the allegory of the cave in Book VII of The Republic. The allegory depicts ordinary people as living locked in a cave, which represents the world of sense-experience; in the cave people see only unreal objects, shadows, or images. But through a painful process, which involves the rejection and overcoming of the familiar sensible world, they begin an ascent out of the cave into reality; this process is the analogue of the application of the dialectical method, which allows one to apprehend unchanging objects and thus acquire knowledge. In the allegory, this upward process, which not everyone is competent to engage in, culminates in the direct vision of the sun, which represents the source of knowledge.
Plato's theory of perception is set out in the Theaetetus and the Timaeus. His view is that the senses provide nothing more than appearances of things which cannot themselves be perceived, that the objects of perception have no fixed natures, and therefore, that what perception gives us (i.e., appearances) cannot be known. The objects of knowledge are the forms. Perceptibles can be partially understood on the supposition that they are the products of interactions between geometrical particles, which constitute the body, and the physical things it confronts. To the extent that these particles resemble the geometrical forms, they can be understood as approximations of facts, which can be inferred from pure geometry.
Aristotle has an opposite view: the objects of knowledge are perceptibles, and abstractions from perceptible objects. Whereas Plato suggested that man was born with knowledge, Aristotle argued that knowledge comes from experience. And there, in the space of just a few decades the two philosophers made up the essence of those two philosophical traditions that have occupied the western intellectual tradition for the past 2500 years. Rationalism - knowledge is a priori (comes before experience) and Empiricism - knowledge is a posteriori (comes after experience). Aristotle argued that there were universal principles but that they are derived from experience. He could not accept, as had Plato, that there was a world of Forms beyond space and time. Aristotle argued that there were Forms and Absolutes, but that they resided in the thing itself. Since the knowledge formulated in syllogisms (a syllogism is, according to the Merriam-Webster Collegiate Dictionary, Tenth Edition, a deductive scheme of a formal argument consisting of a major and a minor premise and a conclusion (as in "every virtue is laudable; kindness is a virtue; therefore kindness is laudable")) resides in the mind, which is part of or one faculty of the soul, much of what Aristotle says about knowledge is part of his doctrine about the nature of soul and, in particular, human soul. As he uses the term, every living thing, including plant life, has a soul (psyche), a soul being what makes a thing alive. Thus it is important not to equate soul with mind or intellect. The intellect (nous) might variously be described as a power, faculty, part, or aspect of the human soul. It should be stressed that for Aristotle the terms soul (psyche) and intellect (nous) and its constituents were understood to be scientific terms. Knowledge is something that a person has. Thus it must be in him somewhere, and the location must be his mind or intellect. Yet there can be no knowledge if the knower and the thing known are wholly separate. The relation between the knowledge in the person or his mind and the object of his knowledge is, Aristotle claims, that "Actual knowledge is identical with its object."
Plato argues that to the extent that humans have knowledge, they attain it by transcending the information provided by the senses in order to discover unchanging objects. But this can be done only by the exercise of reason, and in particular by the application of the dialectical method of inquiry inherited from Socrates. In searching for unchanging objects, Plato begins his quest by pointing out that every faculty in the human mind apprehends a set of unique objects: hearing apprehends sounds but not odours; the sense of smell apprehends odours but not visual images; and so forth. Knowing is also a mental faculty, and therefore there must be objects that it apprehends. These have to be unchanging, whatever they are. Plato's discovery is that there are such entities. Roughly, they are the items denoted by predicate terms in language: such words as "good," "white," or "triangle." To say "This is a triangle" is to attribute a certain property, that of being a triangle, to a certain spatiotemporal object, such as a particular figure drawn on a blackboard. Plato is here distinguishing between specific triangles that can be drawn, sketched, or painted and the common property they share, that of being triangular. Objects of the former kind he calls particulars. They are always located somewhere in the space-time order, that is, in the world of appearance. But such particular things are different from the common property they share. Three triangles, drawn on a piece of paper are particulars that share a common property, triangularity. That common property is what Plato calls a "form" or "idea." Unlike particulars, forms do not exist in the space-time order. Moreover, they do not change. They are thus the objects that one must apprehend in order to acquire knowledge.
In his sketchy account of the process of thinking in De anima (On the Soul), Aristotle says that the intellect, like everything else, must have two parts: something analogous to matter and something analogous to form. The first of these is the passive intellect; the second is active intellect, of which Aristotle speaks tersely. "Intellect in this sense is separable, impassible, unmixed, since it is in its essential nature activity. When intellect is set free from its present conditions it appears as just what it is and nothing more: it alone is immortal and eternal and without it nothing thinks."
By stating that "Actual knowledge is identical with its object" Aristotle meant that knowledge, which must be true and accurate, couldn't deviate from its object in any way. When a person learns something, he acquires something. What he acquires must either be something different from the thing he knows or identical with it. If it is something different, then there is a discrepancy between what he has in mind and the intended object of his knowledge. But such a discrepancy seems to be incompatible with the existence of knowledge. One cannot know that blue is a color if the object of that knowledge is something other than that blue is a color. This idea that knowledge is identical with its object is dimly reflected in the repetition of the variable p in the standard formula about knowledge: S. knows that p just in case it is true that
The unphilosophical man is at the mercy of sense impressions and unfortunately, our sense impressions oftentimes fail us. But because one trust the senses, one is prisoner in a cave and mistakes shadows on a wall for reality (Plato's allegory of the cave - Book VII of The Republic). Because the senses may deceive, it is necessary that this higher world exist, a world of Ideas or Forms -- of what is unchanging, absolute and universal. In other words, although there may be something from the phenomenal world, which we consider beautiful or good, or just, Plato postulates that there is a higher unchanging reality of the beautiful, goodness or justice. To live in accordance with these universal standards is the good life - to grasp the Forms is to grasp ultimate truth.
Plato's search for definitions and thereby the nature of forms is a search for knowledge. But how should knowledge in general be defined? In Theaetetus Plato argues that it involves true belief. No one can know what is false. A person may mistakenly believe that he knows something, which is in fact false, but this is only thinking that one knows, not knowing. Knowledge is at least true belief, but it must also be something more. Suppose that someone believes there will be an earthquake in September because of a dream he had in April and that there in fact is an earthquake in September, although there is no connection between the dream and the earthquake. That person has a true belief about the earthquake but not knowledge. What the person lacks is a good reason supporting his true belief. In a word, the person lacks justification for it. Thus, in Theaetetus, Plato concludes that knowledge is justified true belief.
Although it is difficult to explain what justification is, most philosophers accepted…