It lies in the very nature of epistemology to question the capability of man's mind to contact reality and to know what things are in themselves, the validity of all knowledge, and consequently also of science, it at sake. The foundations of human knowledge are challenged, examined and frequently attacked. An acquaintance with this problem and its possible solution will be, therefore, a matter of prime importance for every seeker of the truth and for every student of philosophy.This is a reprint of the 1936 edition.