"Sitting there on the heather, on our planetary grain, I shra..." — Olaf Stapledon quote meaning

Sitting there on the heather, on our planetary grain, I shrank from the abysses that opened up on every side, and in the future. The silent darkness, the featureless unknown, were more dread than all the terrors that imagination had mustered. Peering, the mind could see nothing sure, nothing in all human experience to be grasped as certain, except uncertainty itself; nothing but obscurity gendered by a thick haze of theories. Man's science was a mere mist of numbers; his philosophy but a fog of words. His very perception of this rocky grain and all its wonders was but a shifting and a lying apparition. Even oneself, that seeming-central fact, was a mere phantom, so deceptive, that the most honest of men must question his own honesty, so insubstantial that he must even doubt his very existence.