"It is important to realize that the exercise of any skill depends on the ability to create an abstract system of some kind out of the totality of the world around us. For instance, the carpenter is not interested in wood as a biological or chemical entity. He is sensitive to many of its grosser physical properties but not to many subtler ones. The wood of a carpenter is not the real — that is, the complete substance — but merely wood as a material on which the carpenter can exercise his skill."