§ 2. When we scrutinise the original “pre-scientific” theory of the world from this point of view, we shall find that its four leading features give rise to four metaphysical problems of great generality and considerable difficulty. The conception of the world as made up of a multiplicity of things, each of which is one, gives rise to the problem of the unity of the thing; the plurality of the qualities, and again of the relations ascribed to the single thing, gives rise to the problems of _Substance_ and _Quality_ and of _Relation_; the belief in the interaction between different things finally gives rise to the exceptionally important and difficult problem of _Causality_. The four problems are not altogether disconnected; in particular, it is hard to discuss the sense in which a thing can be spoken of as “one,” without at the same time raising the question how the “one” thing stands to its many properties, and again discussing the general meaning of relation. And the problem of Causality may be raised in so general a form as to include the other three. Still, for the sake of having a definite order of discussion, it will be well to take them as far as may be separately, and to proceed from the simpler to the more complex. When we have indicated in outline our solution of these problems, we shall have to ask what is the general conception of a thing which our results establish, and whether and on what grounds we are warranted in believing in the actual existence of things answering to our conception. The present chapter will be devoted to the examination of the first three problems; in the succeeding chapter we shall discuss the meaning of Causality, and indicate our general conclusion as to the existence of “things.” With this result our survey of the general structure of Reality will be completed, and we shall then proceed in our third and fourth books to examine the most important of the special problems suggested by the existence of physical nature and conscious mind respectively.