This is Volume IV in a series of twenty-one in a collection on Cognitive Psychology. Originally published in 1933, this looks at the nature of learning in its relation to the living system. In order to discover the mechanism of the living system, itis necessary to investigate which among its effects are connected with well-established laws of chemistry and physics and to distinguish them carefully from the effects which have no immediate, or at least known, relation with these laws, and of which the cause is concealed for us.