It is the sweep of Utopianism, its attempt to deal with society as a whole, leaving no stone unturned. It is the conviction that one has to go to the very root of the social evil, that nothing short of a complete eradication of the offending social system will do if we wish to "bring any decency into the world" (as Du Gard says). It is, in short, its uncompromising radicalism...Both Plato and Marx are dreaming of the apocalyptic revolution which will radically transfigure the whole social world.
This sweep, this extreme radicalism of the Platonic approach (and of the Marxian as well) is, I believe, connected with its aestheticism, i.e. with the desire to build a world which is not only a little better and more rational than ours, but which is free from all ugliness: not a crazy quilt, and old garment badly patched, but an entirely new gown, a really beautiful new world. This aestheticism is a very understandable attitude; in fact, I believe most of us suffer a little from such dreams of perfection. But this aesthetic enthusiasm becomes valuable only if it is bridled by reason, by a feeling of responsibility, and by a humanitarian urge to help. Otherwise it is a dangerous enthusiasm, liable to develop into a form of neurosis and hysteria.
Thoreau famously lamented that there were a thousand people hacking at the branches of evil for every one striking at the root, and in this, as in so many other instances, he showed a talent for crafting vivid metaphors which disguised mistaken ideas. Our minds are pattern-seeking machines, and they love simplicity and clarity. The idea that there is such a thing as a "root" of evil is almost irresistibly perfect. But to continue with the gardening imagery, the best we can ever hope to do is prune the branches. Social ills are rarely cooperative enough to have one simple cause for us to target. Luckily, though, we have the Internet now, where those "dangerous enthusiasms" tend to get safely channeled into ranting like an adolescent who just discovered that his parents weren't kidding when they told him that life is unfair.