At the level of ordinary common-sense thought there appear to be two kinds of Reality--mind and matter. And yet our experience of the unity of Nature, of the intimate connexion between human and animal minds and their organisms (organisms governed by a single intelligible and interconnected system of laws) is such that we can hardly help regarding them as manifestations or products or effects or aspects of some one Reality. There is, almost obviously, some kind of Unity underlying all the diversity of things. Our world does not arise by the coming together of two quite independent Realities--mind and matter--governed by no law or by unconnected and independent systems of law. {21} All things, all phenomena, all events form parts of a single inter-related, intelligible whole: that is the presupposition not only of Philosophy but of Science. Or if any one chooses to say that it _is_ a presupposition and so an unwarrantable piece of dogmatism, I will say that it is the hypothesis to which all our knowledge points. It is at all events the one common meeting-point of nearly all serious thinkers. The question remains, 'What is the nature of this one Reality?' Now, if this ultimate Reality be not mind, it must be one of two things. It must be matter, or it must be a third thing which is neither mind nor matter, but something quite different from either. Now many who will not follow the idealistic line of thought the whole way--so far as to recognize that the ultimate Reality is Mind--will at least admit that Idealists have successfully shown the impossibility of supposing that the ultimate Reality can be matter. For all the properties of matter are properties which imply some relation to our sensibility or our thought. Moreover, there is such a complete heterogeneity between consciousness and unconscious matter, considered as something capable of existing without mind, that it seems utterly impossible and unthinkable that mind should be simply the product or attribute of matter. That the ultimate Reality cannot be what we mean by matter has been admitted by the most naturalistic, {22} and, in the ordinary sense, anti-religious thinkers--Spinoza, for instance, and Haeckel, and Herbert Spencer. The question remains, 'Which is the easier, the more probable, the more reasonable theory--that the ultimate Reality should be Mind, or that it should be something so utterly unintelligible and inconceivable to us as a _tertium quid_--a mysterious Unknown and Unknowable--which is neither mind nor matter?' For my own part, I see no reason to suppose that our inability to think of anything which is neither matter nor mind but quite unlike either is a mere imperfection of human thought. It seems more reasonable to assume that our inability to think of such a mysterious X is due to there being no such thing.[7]