For much of the twentieth century, most scientists assumed that a human being is simply a combination of ordinary physical elements. In Human Devolution, Cremo says it is more reasonable to assume that a human being is a combination of three distinct substances: matter, mind, and consciousness (or spirit). He shows how solid scientific evidence for a subtle mind element and a conscious self that can exist apart from the body has been systematically eliminated from mainstream science by a process of “knowledge filtration.”