Is rationalism plausible?

Rationalism is founded on an extraordinary assumption: that human reason has within itself the resources for discerning and understanding reality's deepest fixtures. It is easy to imagine this assumption being false. Perhaps human reason is simply incapable of working out the deepest truths about reality, or perhaps we can do so only through extensive empirical investigation. Perhaps human reason evolved under prehistoric pressures, and we are very talented at building fires and catching rabbits but very poor at doing metaphysics. Perhaps there is no such thing as ultimate reality, as crazy at that may sound, and all human experience is nothing but interpretations upon interpretations. Perhaps the biggest truths are inconceivable. All of these are possibilities, but rationalism denies them. As said in the introduction, rationalism holds that the innermost skeleton of reality and the innermost skeleton of the human mind are one and the same.

And what do our rationalists conclude? Descartes concludes that there are two radically different kinds of substance in the world, minds and bodies, and that in every person there is one of each in mysterious interaction. Spinoza concludes that there can be only one substance, which is both thinking and extended, and that the apparent individuals in the universe are in fact particular ways in which that one substance is expressed. Leibniz concludes that there are infinitely many minds which mirror one another in such a way as to produce the appearance of a shared world of bodies in motion.