Digital Companion to C. S. Peirce
Some Wit, Wisdom & Bewilderment
A man may walk down Wall Street debating within himself the existence of an external world; but if in his brown study he jostles up against somebody who angrily draws off and knocks him down, the sceptic is unlikely to carry his scepticism so far as to doubt whether anything beside the ego was concerned in that phenomenon. The resistance shows him that something independent of him is there.
The Logic of Mathematics, c. 1896