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 strange to us that what the philosopher terms an ‘idea’ is a process which can cause chemical changes in the body.

Through the discovery of Buchner, Biology was relieved of another fragment of mysticism. The splitting up of sugar into CO2 and alcohol is no more the effect of a ‘vital principle’ than the splitting up of cane sugar by invertase. The history of this problem is instructive, as it warns us against considering problems as beyond our reach because they have not yet found their solution.