Although Plato and Aristotle lived in the same period, the two philosophers developed two divergent theories of knowledge. In order to define knowledge, Plato uses his Theaetetus dialogue, particularly a conversation between Socrates and Theaetetus about knowledge, the divided line diagram, and the allegory of the cave. In the Theaetetus dialogue, Plato introduces the three definitions of knowledge proposed by Theaetetus. He, Theaetetus, declares, under the leadership of Socrates, that knowledge is perception [Aisthetes] or, as Protagoras expresses it, that “man is the measure of all things”. Socrates rejects the first proposition stating that if man is the true measure of all things and his perception is infallible; thus making man the sole judge of what is good and evil. If man is the only judge, then there is no need for a teacher who claims that man is the measure of all things. Theaetetus states that knowledge is true opinion. However, Socrates also refutes this claim, saying that it is impossible to distinguish a true opinion from a false opinion, if it is the opinion itself that is used as the distinguishing factor. Theaetetus then suggests that knowledge is true opinion with explanation. This definition is also rejected by Socrates, who argues that if the explanation is what makes the opinion true, then that opinion is not necessary. With these rejected definitions in mind, Plato's theory of knowledge, based on idealism, calls for the separation of form and matter as well as the existence of two worlds: the world of appearances and the world of the intelligible. The Platonic theory of knowledge states that knowledge is acquired through the ascent of what Plato calls the Plateaus of Knowledge...... middle of paper ...... being subjects while accidental things are limited to being only predicates. Substances are the “proper category of unqualified science” and answer the question “What thing is it?” The nine accidents: Quantity, Quality, Relationship, Place, Time, Situation, State, Action and Passion, are used to classify knowledge, essentially propositions about aspects of things, provided first by experience and then by art. Plato and Aristotle propose theories of knowledge in which they both agree that the knower is measured by the known and that knowledge is an exchange within the world. However, their respective theories can be seen as polar opposites of each other, especially considering that Aristotle rejects Plato's theory and admits that "enlightened opinion" is a form of knowledge while Plato rejects opinion as a form of knowledge..