Date Available


Year of Publication


Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation


Arts and Sciences



First Advisor

Dr. Eric Sanday


I will argue that Plato’s dialogue Theaetetus demonstrates that knowledge is never caused by sense perception. While various kinds of qualities appear to the soul or mind as a result of sense perception—as a result of external bodies impacting with the sense organs—the being (einai or ousia) of these qualities is something different from the mere appearance of the qualities that occurs through the senses. While white colors appear to the soul through vision, perception itself does not reveal that these many appearances are all instances of one white quality. However, I will demonstrate that it is impossible to know anything, even something as basic as “the chalk is white,” if the knower does not recognize that “white” is some one thing and not merely a plurality of instances. Since sense perception does not disclose the one being of what appears within it, knowledge always requires the soul or mind to go beyond what is receptively disclosed to it through sense perception.

In order to demonstrate this conclusion, Plato uses a reductio ad absurdum argument. He develops a theory that argues for the opposite conclusion. According to this theory, perceiving and knowing are the same. In order to justify this result, the theory posits that qualities have no one being that is distinct from their many appearances. I will show that the theory entails a series of unacceptable consequences. The worst of these consequences is that it makes reality itself is unintelligible—according to the theory, the world cannot be linguistically described because the world does not possess any concrete determinacy to describe as a result of the theory denying the difference between being and appearances. Plato’s Socrates demonstrates that these conclusions are unacceptable on the theory’s own terms. As a result, the theory fails and the postulate that being and appearances are identical must be rejected. It is impossible for the mere appearance of a quality through sense perception to ever be knowledge. It will only be possible for knowledge to come about through an activity of the soul that discovers the being of what appears to it.