In his Socratic dialogues The Phaedo and The Meno, Plato advances a theory concerning the acquisition of human knowledge. In these dialogues, Plato asserts that people acquire knowledge through recollection; that is, nothing is learned new, from experience. Knowledge exists a priori in the human soul, and while certain experiences may trigger the recollection of a priori truths, knowledge comes not from that experience itself but from the remembrance of what was within all along. Later, in his Metaphysics, Aristotle advanced a conception of the universe and human knowledge which was heavily critical of this theory of recollection.
In the Phaedo, Socrates illustrates the theory of recollection by referring to sticks of equal length. According to him, sticks are never perfectly equal. If they appear equal to me, they may not appear so to you. If they are equal in length, they may not be equal in width. Finally, if they appear to be equal to anyone, they probably aren't precisely equal, but are, in fact, merely extremely close to being equal. Using a more precise instrument of measurement may reveal the minute, perhaps invisible differences in the two sticks' length. However, Socrates says, when someone realizes that the two sticks are not quite equal, "he, in realizing this, must have foreknowledge of that to which he says the thing is alike but falling short of". In other words, the observer realizes that the two sticks' relation to one another falls short of the ideal Form of the Equal itself, but, in doing so, reveals that they must have some knowledge of this Form of the Equal itself. However, the ideal Forms (of which "Equality" is just one) have no manifestation in the physical realm. The observer can never have directly observed a Form, and yet the observer, in recognizing the variety or imperfections of the physical world, reveals themselves to have knowledge of Forms. This knowledge, again, cannot have come from experience. Therefore, any human knowledge of Forms is revealed to be a priori. As Socrates says, "â€¦we must have possessed knowledge of what the Equal itself is if we were about to refer our sense perceptions of equal objects to itâ€¦". This knowledge must have had its source prior to the beginning of our sensory perceptions, simply because, as we know from experience, we do not experience Forms themselves. Thus this knowledge must have been somehow implanted into our soul prior to our birth.
In the Meno, the Sophist Meno alleges that Socrates' search for knowledge is a fruitless one, because, if Socrates truly does not know that which he searches for, he will never be able to know it simply because if true knowledge is presented to him, he will not "know that it is the thing which he did not know". Socrates, however, attempts to demonstrate how the theory of recollection circumvents this difficulty. In the theory of recollection, one does not truly move from no knowledge to knowledge. Instead, experience triggers recollection of truth that was internal and existed within the soul prior to the experience. Thus, knowledge is present all along, but buried or lost within the mind.
Aristotle's conception of nature, as presented in his Metaphysics, is highly opposed to the Platonic doctrine of forms in general and the accompanying theory of recollection. In the picture Aristotle paints, the Universal (what Plato would have termed the Form or the Ideal) is present within all of the particulars. The universal is only to be determined or discovered through a careful study of all of the particulars which compose it. Thus, it may be seen from this picture, knowledge, for Aristotle, is not a priori, but rather relies upon experience. Only by experiencing many different manifestations of types of objects may one come to the sorts of truths which may be generalized to apply to all particular instances.
Aristotle addresses this issue in the first book of the Metaphysics. As he explains it, there are several different types of knowledge. Men who know the art of a thing, that is, men who have true knowledge of a thing, "know the why of it or the cause". Men who have experience do not know the cause of things, but may still know the facts of a particular case. However, Aristotle makes it clear that true "art" may only be originally derived through multiple experiences of particular instantiations of a particular phenomenon. By gathering information from these multiple experiences, one may come to piece together a theory which explains individual events.
Aristotle takes his primary example of this from medicine. A doctor who treats someone with a fever knows how to treat the fever only because of repeated experience with others who have had fevers. Because the doctor's patients in the past have been human beings, and because the doctor's present patient is also a human, the doctor has experiences with patients upon which to draw for information as to how to treat the present patient. A doctor who is truly knowledgeable, that is, a doctor who is possessed of the art of medicine, knows, besides precisely how to treat the fever, why the fever is occurring. However, the only way someone can ever come to discover why a fever occurs is by studying fevers in many similar beings, in this case, humans. This knowledge may be transmitted through teaching after it is discovered, but it may only be discovered originally through intensive study of particular phenomena.
Aristotle is thus revealed to be an empiricist. Knowledge is only to be gained, in Aristotle's philosophical system, through the repeated observation of physical phenomena. In Plato's system, however, knowledge exists before the experience of physical phenomena, and while the experience of physical phenomena may trigger knowledge, such experience does not create knowledge, but merely triggers the remembrance of a priori knowledge of forms.