For the naturalist everything lies in the method, in the chance of obtaining an unshakeable, lasting truth; and solely from this point of view, which for him is obligatory, the soul, as a naturalistic principle, is not only unnecessary but even harmful to his work, in vain limiting his courage and the depth of his analysis.

Experimental Psychology and Other Essays, Experimental Psychology and Psychopathology in Animals (p. 168), Philosophical Library. 1957