"The nature, functions, and behaviour of ideas, and how ideas have power in their impact upon the cerebral hemisphere to make some sort of sensible impression––all this is matter as to which we are able only to make 'guesses at truth.' But this need not dismay us, for such other ultimate facts as sleep and life and death are equally unexplained. In every department of science we are brought up before facts which we have to assume as the bases of our so-called science. Where a working hypothesis is necessary, all we can do is to assume those bases that seem to us the most adequate and the most fruitful."
CM Series Volume 3 page 69