In my interpretation, Karl Lashley's life and work illustrate three basic themes that I want now to review and to tie together. The first is the idea that during the period from 1910 to 1955, the mind-body problem – the puzzle of the relationship between consciousness and matter – became inseparably involved with the nature-nurture dispute: which was the greater determinant of intelligence and ability, heredity or environment? It is difficult to find a life scientist from the first half of this century who did not express an opinion on the nature-nurture question, especially as genetics developed in scientific and social significance, and the question took on increasing urgency. As a result, understanding psychologists' and biologists' answers to the problem of mind, brain, and behavior is impossible unless those answers are seen in the light of the nature-nurture debate.