We believe our bodies to be real and natural, that they exist in a particular way in objective time and space without our having to think about them, and that if we collectively put our minds to the task, we can know and understand them as they actually function, just as we know the world in which it all takes place. In short, when we examine the assumptions underlying common everyday existence and action we discover something very close to the assumptions of philosophical materialism. It is important to emphasize that materialism does not deny the mental or spiritual aspects of our lives. The mental is regarded as fully real, but the material is prior. This priority, however, must be understood developmentally. It is certainly not the case that thoughts never precede or have effects upon material processes. On the contrary, thoughts and other mental phenomena almost always affect material processes. But these same mental processes have their ultimate origin in material, usually biological processes. They are themselves material processes of a developmentally special kind.