I believe that the investigation of the conditions which produce tropisms may be of importance for psychiatry. If we can call forth in an animal otherwise indifferent to light by means of an acid a heliotropism which drives it irresistibly into a flame; if the same thing can be brought about about by means of a secretion of the reproductive glands; then we have given, I believe, a group of facts, within which the analogies necessary for psychiatry can be experimentally called forth and investigated.
These experiments may also attain a similar value for ethics. The highest manifestation of ethics, namely, the condition that human beings could be willing to sacrifice their lives for an idea is comprehensible neither from the utilitarian standpoint nor from that of the categorical imperative. In this case also it might possibly be that under the influence of certain ideas chemical changes, for instance, internal secretions within the body, might be produced which increase the sensitiveness to certain stimuli to such an unusual degree that such people become slaves to certain stimuli just as the copepods become slaves to the light. To-day, since Pawlow and his pupils have succeeded in causing the secretion of saliva in the dog by means of optic and acoustic signals, it no longer seems to us so strange that what the philosopher terms an "idea" is a process which can cause chemical activity in the body.