On Public and Private Criteria

“One can indeed prove something to be real by appealing to the experiences and testimony of others, but the existence of such experience and such testimony, and of such others, must be something whose tests lie, or could lie, in a man’s own experience, and of whose validity he alone is the final judge. A number of persons can entertain and verify some hypothesis in a co-operative manner, performing the same experiments, witnessing their outcome and drawing the same conclusions, but they can do so only because each, with his experiences, is given in the experiences of each other, so that, while all carry out common tests, the tests are also carried out by each man severally.” Transcendence of the Cave, p. 29-30)