As to the number and nature of the supposed independent real things, very different views may be held and have been held by different representatives of Realism. Thus some realists have maintained the existence of a single ultimate reality, others of an indefinite plurality of independent “reals.” Parmenides, with his doctrine that the real world is a single uniform unchanging material sphere, is an instance in the ancient, and Mr. Herbert Spencer, with his Unknowable, an instance in the modern world of a realist of the former or “monistic” type. The ancient atomists, and in more recent times Leibnitz with his infinite plurality of independent and disconnected monads, and Herbart with his world of simple “reals,” afford the best known instances of a doctrine of pluralistic Realism. So again, the most diverse theories have been propounded as to the nature of the “reals.” Ancient and modern atomists have thought of them as material, and this is perhaps the form of realistic doctrine which appeals most readily to the ordinary imagination. But though a materialistic metaphysician is necessarily a realist, a realist need not be a materialist. Herbart thought of the independent “reals” as qualitatively simple beings of a nature not capable of further definition, Leibnitz as minds, while Agnostic philosophers of the type of Spencer conceive their ultimate reality as a sort of neutral _tertium quid_, neither mental nor material. The only point on which all the theories agree is that the reality of that which they recognise as true Being consists in its not depending for its existence or its character on relation to an experience. The differences of detail as to the number and nature of independent “reals,” though of great importance for our complete estimate of an individual realist’s philosophical position, do not affect our general verdict on the tenability of the first principle of Realism. The one point of divergence among realists which may be considered as of more than secondary importance for our present purpose, is the difference between what we may call Agnostic and Dogmatic Realism. Agnostic Realism, while asserting the ultimate dependence of our experience upon a reality which exists independently of experience, denies that we can have any knowledge of the nature of this independent reality. The independent reality by which all experience is conditioned is, on this view, an Unknowable or Thing-in-itself,[43] of which we are only logically entitled to say _that_ it certainly is, but that we do not in the least know _what_ it is. The doctrine of Agnostic Realism has probably never been carried out by any thinker with rigid consistency, but it forms a leading feature of the philosophy of Kant as expounded in his First Critique, and through Kant has passed into English thought as the foundation of the systems of Sir William Hamilton and Mr. Herbert Spencer.[44]