Consider these quotations from three philosophers you probably aren’t inclined to group together:
The light dove, cleaving the air in her free flight, and feeling its resistance, might imagine that its flight would be still easier in empty space. It was thus that Plato left the world of the senses, as setting too narrow limits to the understanding, and ventured out beyond it on the wings of the ideas, in the empty space of the pure understanding…He did not observe that with all his efforts he made no advance – meeting no resistance that might, as it were, serve as a support upon which he could take a stand (Kant, Critique of Pure Reason, Intro, Sec. 3).
Because we do not know the essence of God, the proposition is not self-evident to us; but needs to be demonstrated by things that are more known to us, though less known in their nature — namely, by effects…Granted that everyone understands that by this word “God” is signified something than which nothing greater can be thought, nevertheless, it does not therefore follow that he understands that what the word signifies exists actually, but only that it exists mentally. Nor can it be argued that it actually exists, unless it be admitted that there actually exists something than which nothing greater can be thought; and this precisely is not admitted by those who hold that God does not exist (Aquinas, Summa Theologiae I.2.1.c., ad 2).
The more closely we examine actual language, the greater becomes the conflict between it and our requirement. (For the crystalline purity of logic was, of course, not something I had discovered: it was a requirement.) The conflict becomes intolerable…We have got on to slippery ice where there is no friction, and so, in a certain sense, the conditions are ideal; but also, just because of that, we are unable to walk. We want to walk: so we need friction. Back to the rough ground! (Wittgenstein, Philosophical Investigations, Sec. 107).
All three thinkers are combatting an extreme optimism about the uses of a priori reasoning: for Kant, Plato’s; for Aquinas, (probably) Anselm’s; for Wittgenstein, possibly that of his earlier self, as author of the Tractatus. In all three cases, their point is that all thought requires, for its intelligibility, some restraining limits, whether of everyday experience (Aquinas and Wittgenstein) or of the transcendental conditions of thought itself (Kant).