§ 4. _The Problem of Substance and Quality._—More important, in the history of metaphysical theory, has been the other aspect of the problem of the unity of things. What we call one _thing_ is said, in spite of its unity, to have many _qualities_. It is, _e.g._, at once round, white, shiny, and hard, or at once green, soft, and rough. Now, what do we understand by the _it_ to which these numerous attributes are alike ascribed, and how does it possess them? To use the traditional technical names, what is the _substance_ to which the several qualities belong or in which they inhere, and what is the manner of their _inherence_? The full difficulty of this problem may be most easily exhibited by considering the ways in which popular thought commonly tries to solve it. (1) One of the commonest and most obvious solutions is to identify the “substance” which has the qualities (or, to use the more general scholastic expression, the _accidents_) with some one group of the thing’s properties which we regard as specially important or permanent. The “substance” is then taken to be just this group of “primary” qualities, and is said to have or possess the less permanent “secondary qualities.” For obvious reasons, the “primary” qualities have in modern Philosophy usually been identified, as by Galileo, Descartes, and Locke, with those mathematical properties of body which are of fundamental importance for the science of mechanical Physics.[76] And usually, though not always, the way in which the substance, as thus defined, _has_ the secondary qualities, has been further explained by saying that these latter are subjective changes in our sensibility produced by the action of the primary qualities upon our various sense-organs. Neither of these special views is, however, necessarily involved in the identification of the substance of things with their fundamental qualities. The essential principle of the theory consists simply in the recognition of some groups of qualities as of primary importance, and the identification of the one “substance” which has the many properties with this group.