The words “mind” and “matter” are used glibly, both by ordinary people and by philosophers, without any adequate attempt at definition. Philosophers are much to blame for this. My own feeling is that there is not a sharp line, but a difference of degree; an oyster is less mental than a man, but not wholly un-mental. And I think “ mental ” is a character, like “ harmonious ” or “ complicated,” that cannot belong to a single entity, but only to a system of entities. But before defending this view, I wish to spend some time on traditional theories.