Bertrand Russell's "A History of Western Philosophy" - The Idealists and Prejudicing Permanence

For Plato and others like him, there is the illusory sensible world of appearance, and there is the eternal world from which the sensible word emanates. Nothing in the sensible world can be "perfect" in the sense that there is nothing permanent or absolutely rational in the sensible world. We cannot perceive a perfectly straight line through our bodily senses. However, we can conceive of—or inwardly perceive—a perfectly straight line in our minds.

But why should the rationally perfect concepts of the ideal world have any more reality than the alleged imperfection manifestations of those concepts? Plato's idealism strikes me as a sort of universal solipsism: the world emanates from the ideas in our mind.

I don't think that there's a way to persuasively prove Plato is absolutely incorrect on this point. However, his correctness is irrelevant. What is relevant is the attitude that Platonic Idealism leads to. The external world (sensible world) is constructed as a bunch of irrelevant appearances. The internal world (ideal world) is the only place where permanence, rationality, and happiness can be found.

But why should the rational or the permanent be valued any more highly than the irrational and the fickle? Perhaps many people are attracted to that which is permanent and consistent, because it is easier to feel safe and happy in a world with few surprises. This is not true of all people, of course, and it is not true of all people over the course of their entire life. But it seems to me that the valuation of that which is permanent over that which is transient is entirely arbitrary, and a matter of individual temperament.

The ideas of "truth" and "permanence" are often clustered together. For example, when we say that "It's absolutely true that 2+2 = 4," most of us probably believe that the truth of the statement is permanent—we believe that 2+2 will not equal 5 tomorrow, or that 2+2 did not equal 3 yesterday.

However, it's perfectly possible to conceive of this truth as being impermanent. Imagine that we all were to wake up tomorrow, and find that 2+2 = 5. When we count on our fingers, in our minds, and using calculators, it all adds up to 5. All of our computation and every piece of technology that relies upon their successful calculation will not execute properly unless the sum of 2 and 2 is valued at 5. Our own sense of internal logic believes that 2+2 = 5, and it seems absurd that we have spent so much time believing that 2+2 = 4.

It's unlikely that this is going to happen. But it's conceivable—that is to say, we can imagine it happening.