H. P. Lovecraft — The Dreams in the Witch House

I just finished the third and final volume of Penguin's collection of Lovecraft and loved all of it. So much of Lovecraft's writing is about creating a mounting atmosphere of dread, horror, wonder or despair that it's not easy to pick just one passage to give a good sense of his writing. That being said, this one does a suitable job of describing his opinion of humanity and its place in the cosmos:

He had read much of things as they are, and talked with too many people. Well-meaning philosophers had taught him to look into the logical relations of things, and analyse the processes which shaped his thoughts and fancies. Wonder had gone away, and he had forgotten that all life is only a set of pictures in the brain, among which there is no difference betwixt those born of real things and those born of inward dreamings, and no cause to value one above the other. Custom had dinned into his ears a superstitious reverence for that which tangibly and physically exists, and had made him secretly ashamed to dwell in visions. Wise men told him his simple fancies were inane and childish, and even more absurd because their actors persist in fancying them full of meaning and purpose as the blind cosmos grinds aimlessly on from nothing to something and from something back to nothing again, neither heeding nor knowing the wishes or existence of the minds that flicker for a second now and then in the darkness.

From The Silver Key

And this:

The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far. The sciences, each straining in its own direction, have hitherto harmed us little; but some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the light into the peace and safety of a new dark age.

From The Call of Cthulhu