For example something at time t1 can appear to be hot, whereas at t2 can appear to be cold. With perception, it is true to say that everything must be in flux and truth something subjective. Plato does not disagree with this, rather he disagrees that this is the complete description of what we mean by knowledge and that if we take it as the absolute description of what is, rather than merely the description of the act of perception in a restricted sense, we will be lead to absurdities. What this picture leaves out is that we do not merely perceive the world, but we use concepts to describe it, and concepts are not themselves something that can be perceived. To understand the difference between a perception and a concept means understanding the difference between white as a property of a thing and whiteness itself.
Innumerable things can be white, but nothing can be whiteness itself.
I perceive a white stick or a white stone, but I do not perceive whiteness itself and yet I do know what whiteness means, because I understand what the word expresses, even though I have never seen whiteness itself. Now it is upon these general concepts that my judgements are based not upon my subjective perceptions and it is because of these general concepts that we have a conception of truth and falsity, because we have an objective measure of what might be right and wrong. The evidence is what I actually do. For although it seems common sense to say that my knowledge about the world is through the senses, this world would in fact be meaningless. The point is I do not just perceive the world, but I reflect upon the perceptions that I have about the world, and I say this stone is white or this stick is white .