How many things are there? Or how many kinds of thing? Monism is the doctrine that the answer to one or other of these questions is "Only one." Opposed to monism is the doctrine of pluralism, which is that there are many kinds of thing, or that there are many things. It will be apparent, on reflection, that this weaker form of pluralism, that there are many things, is quite consistent with the weaker form of monism, that there is only one kind of thing to which the many particular things belong. For instance, materialism, in the sense that everything existent is material, is a form of monism because it insists that all existent things are of a single kind, the material kind. Thus monism and pluralism, though opposed, do not always exclude each other.

A doctrine that might be regarded as a form...