It is a scandal, but that's the way it is.) Plato's point is that we can never have true knowledge of anything that is in a constant state of change. We can only have opinions about things that belong to the world of the senses, tangible things. We can only have true knowledge of things that can be understood with our reason.
All right, Sophie, I'll explain it more clearly: a gingerbread man can be so lopsided after all that baking that it can be quite hard to see what it is meant to be. But having seen dozens of gingerbread men that were more or less successful, I can be pretty sure what the cookie mold was like. I can guess, even though I have never seen it. It might not even be an advantage to see the actual mold with my own eyes because we cannot always trust the evidence of our senses. The faculty of vision can vary from person to person. On the other hand, we can rely on what our reason tells us because that is the same for everyone.
If you are sitting in a classroom with thirty other pupils, and the teacher asks the class which color of the rainbow is the prettiest, he will probably get a lot of different answers. But if he asks what 8 times 3 is, the whole class will--we hope--give the same answer. Because now reason is speaking and reason is, in a way, the direct opposite of "thinking so" or "feeling." We could say that reason is eternal and universal precisely because it only expresses eternal and universal states.
Plato found mathematics very absorbing because mathematical states never change. They are therefore states we can have true knowledge of. But here we need an example.
Imagine you find a round pinecone out in the woods. Perhaps you say you "think" it looks completely round, whereas Joanna insists it is a bit flattened on one side. (Then you start arguing about it!) But you cannot have true knowledge of anything you can perceive with your eyes. On the other hand you can say with absolute certainty that the sum of the angles in a circle is 360 degrees. In this case you would be talking about an ideal circle which might not exist in the physical world but which you can clearly visualize. (You are dealing with the hidden gingerbread-man mold and not with the particular cookie on the kitchen table.)
In short, we can only have inexact conceptions of things we perceive with our senses. But we can have true knowledge of things we understand with our reason. The sum of the angles in a triangle will remain 180 degrees to the end of time. And similarly the "idea" horse will walk on four legs even if all the horses in the sensory world break a leg.
An Immortal Soul
As I explained, Plato believed that reality is divided into two regions.
One region is the world of the senses, about which we can only have approximate or incomplete knowledge by using our five (approximate or incomplete) senses. In this sensory world, "everything flows" and nothing is permanent. Nothing in the sensory world is, there are only things that come to be and pass away.
The other region is the world of ideas, about which we can have true knowledge by using our reason. This world of ideas cannot be perceived by the senses, but the ideas (or forms) are eternal and immutable.
According to Plato, man is a dual creature. We have a body that "flows," is inseparably bound to the world of the senses, and is subject to the same fate as everything else in this world--a soap bubble, for example. All our senses are based in the body and are consequently unreliable. But we also have an immortal soul--and this soul is the realm of reason. And not being physical, the soul can survey the world of ideas.
But that's not all, Sophie. IT'S NOT ALL!