In accepting the invitation kindly given me in this Journal last October to explain more fully my views on the relations of the nervous system to the operations of consciousness, I feel that I labour under more than one difficulty. The questions raised are not to be solved in the main by experiment, though the biologist of the present day is too liable to take for granted that his science can be forwarded by observation and experiment alone, and that there is no art required to draw just conclusions from these. Then for the most part my statements remain unassailed, and the rôle left for me seems to be principally one of reiteration and re-attack. On one point I cannot too much insist, namely, that to prove one theory false it is not necessary to be prepared with another which is true to replace it. The question whether the prevalent theory is correct must not be confused with any other; and I submit that the objections against the current doctrine of sensation, to which I gave publicity at the Liverpool meeting of the British Association in 1870, remain unanswered, not because they are unknown, but because, as I have found physiologists are ready to own, they are incapable of refutation.