The definition of knowledge is key to the search for truth as truth is the outcome of the correct application of knowledge.
Theaetetus thought that one who knows something is perceiving the thing that he knows, and, so far as he could see at the time, knowledge is nothing but perception. This definition suggests that there is no 'one truth' for anything in flux. For example, a man can not say that what he sees, hears, tastes, touches or smells is truth for this is dependent on perception.
However, concepts are universal truth. 2+2 only =4 because it is a product of a concept. It is not dependant on perception and therefore can not be tainted with falsehood.
Heraclitus backs up this thesis by stating...'A thing may change of quality, and the doctrine of flux is held to state that everything is always changing all its qualities. This has awkward consequences. We cannot be right in saying we are seeing a thing, for seeing is perpetually changing into not-seeing. If everything is changing in every kind of way, seeing has no right to be called seeing rather than not seeing, of perception to be called perception rather that not-perception. And when we say 'perception is knowledge', we might just as well say 'perception is not-knowledge'.
What the above argument amounts to is that, whatever else may be in perpetual flux, the meanings of words must be fixed, at least for a time, since otherwise no assertion is definite, and no assertion is true rather than false, condemning knowledge a non-entity.
'Plato goes one step further as he argues that we percieve through eyes and ears, rather than with them. Knowledge is not connected with any sense-organ. We can know, for instance, that sounds and colours are unlike, though no organ of sense can perceive both. There is no special organ for existance and non-existance, likeness and unlikeness, sameness and differences, and also unity and numbers in general. The same applies to GOOD AND BAD. The mind contemplates some things through its own instrumentality, others through the bodily faculties. We perceive hard and soft through touch, but it is the mind that judges that they exist and that they are contraries. Only the mind can reach existence, and we cannot reach truth if we do not reach existence. It follows that we cannot know things through the senses alone, since through the senses alone we cannot know that things exist.Therefore knowledge consists in reflection, not in impressions, and perception is not knowledge, because it has no part in apprehending truth since it has none in apprehending existence.'1
So, under the assumption that the only truth possible is that of a concept, we then need to differenciate concepts and percepts.
1.Bertrand Russel, History of Western Philosophy, pp.166.
