The brain has still another important characteristic. After it has been stimulated through sense organ and nerve, a similar brain activity can be revived later, and this revival is the basis of memory. When the brain is agitated through the medium of a sense organ, we have sensation; when this agitation is revived later, we have a memory idea. A study of consciousness, or mind, the conditions under which it arises, and all the other problems involved, give us the other part of the field of psychology.