Plato's Theaetetus: What Is Knowledge? › The Third Definition: Knowledge Is True Belief with an Account

Theaetetus, prompted by something he once heard, offers a third definition: knowledge is true belief accompanied by an account or explanation (*logos*). This is remarkably close to the "justified true belief" definition that dominated epistemology until the twentieth century. To know something, you must believe it, it must be true, and you must be able to give reasons for your belief. But what exactly is an *account*? Socrates considers three interpretations, and finds problems with each. The first interpretation: an account is putting one's thought into words. But this cannot distinguish knowledge from mere true belief. Anyone with true belief can express it in words. The person who truly believes that Theaetetus is sitting can say "Theaetetus is sitting." Adding words does not transform belief into knowledge. The second interpretation: an account is an enumeration of the elements that compose something. To know a wagon, you list its parts (wheels, axle, body, and so on). To know a word, you spell out its letters. This seems more promising, since analysis reveals structure, but Socrates finds a problem. Consider learning to spell. When you learn the word "Theaetetus," you learn that it is composed of certain letters in a certain order. But do you know each letter? If knowing something requires knowing its elements, and letters are elements, then you must know the letters. But letters have no further elements to enumerate. Either you know them without an account (which contradicts the definition), or you cannot know them at all (which makes knowledge of words impossible). This is the problem of first elements. Any analysis must bottom out somewhere in things that cannot themselves be analyzed. If knowledge requires an account, and an account requires breaking things into parts, then the ultimate parts (the elements) cannot be known. But if the elements cannot be known, how can the wholes composed of them be known? The third interpretation: an account is stating what distinguishes the thing from everything else. To know Theaetetus is to identify what makes him different from all other people (his snub nose, his prominent eyes, and so on). This seems to capture what we want: genuine understanding of the individual thing. But Socrates argues this interpretation is circular. To have true belief about Theaetetus, you must already be thinking about *him* rather than someone else. Your belief must already pick him out. If it does not, you do not have true belief about Theaetetus at all; you have true belief about someone else. So the distinguishing features are already implicit in the true belief. Adding them explicitly as an "account" adds nothing new. ::: {.callout-tip title="Practical Application"} The problem of distinguishing marks appears whenever we try to define something precisely. Job descriptions list required qualifications, but the best candidates often have something indefinable that sets them apart. Character references try to capture what makes a person unique, but the most important qualities resist enumeration. Plato's puzzle suggests that genuine knowledge of individuals may require direct acquaintance rather than descriptions alone. ::: With all three interpretations of "account" failing, the third definition collapses. The dialogue ends without a positive answer to its central question.