It is a life-preserver by which some set great store, but which no man dreams of using when he really falls into the water.
If, then, we have any reason to believe that there is a real external world at all, we have reason to believe that we know what it is.

That some know it imperfectly, that others know it better, and that we may hope that some day it will be known still more perfectly, is surely no good reason for concluding that we do not know it at all.
[1] "First Principles," Part I, Chapter IV, section 26..