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One implication of this definition is that one can't be said to "know" something just because one believes it and that belief turns out to be true. An ill person with no medical training but a generally optimistic attitude might believe that she will recover from her illness quickly. But even if this belief turned out to be true, on the Theaetetus account the patient did not know that she would get well because her belief lacked justification.
There are, according to this account, three categories of belief which are not knowledge: beliefs which are true but not justified; beliefs which are justified but not true (sometimes referred to as Justified Error); and beliefs which are neither justified nor true (sometimes referred to as Unjustified Error).
In the context of epistemology, belief is not used in the everyday sense of having confidence or faith in something. Belief is used in the sense of asserting the truth of some proposition or statement. Beliefs in this sense are either true or false. If Jenny believes that x is true, and x is in fact true, then Jenny holds a true belief. But on the Theaetetus account, if that belief is to count as knowledge, it must also have a suitable justification. Knowledge, therefore, is distinguished from true belief by its justification, and much of epistemology is concerned with how true beliefs might be properly justified. This is sometimes referred to as the theory of justification.
The Theaetetus definition agrees with the common sense notion that we can believe things without knowing them. Whilst knowing p entails that p is true, believing in p does not, since we can have false beliefs. It also implies that we believe everything that we know. That is, the things we know form a subset of the things we believe.
For most of philosophical history, "knowledge" was taken to mean belief that was justified as true to an absolute certainty. Any less justified beliefs were called mere "probable opinion." This viewpoint still prevailed at least as late as Bertrand Russell's early 20th century book The Problems of Philosophy. In the decades that followed, however, philosophers came to think of knowledge as meaning "justified true belief," and the notion that the belief had to be justified to a certainty was forgotten. In the 1960s, Edmund Gettier criticised this definition of knowledge by pointing out situations in which a believer has a true belief justified to a reasonable degree, but not to a certainty, and yet in the situations in question, everyone would agree that the believer does not have knowledge.
The problems show that there are situations in which a belief may be justified and true, and yet most would not consider it to be knowledge. Although being a justified, true belief is necessary for a definition of knowledge, it is not sufficient. At the least, the set of our justified true beliefs contains things that we would not say that we know.
Some epistemologists have attempted to find strengthened criteria for knowledge that are not subject to the sorts of counterexamples Gettier and his many successors have produced. No one has yet succeeded in doing so. Kirkham (see the References section below) has argued that this is because the only definition that could ever be immune to all such counterexamples is the original one that prevailed from ancient times through Russell: to qualify as an item of knowledge, a belief must not only be true and justified, the evidence for the belief must necessitate its truth. But this conclusion is resisted since it would probably entail a sweeping skepticism.