1. _Of Primary and Secondary Qualities._--IN the same way in which the mechanical sciences depend upon the Idea of Cause, and have their principles regulated by the development of that Idea, it will be found that the sciences which have for their subject Sound, Light, and Heat, depend for _their_ principles upon the Fundamental Idea of Media by means of which we perceive those qualities. Like the idea of cause, this idea of a medium is unavoidably employed, more or less distinctly, in the common, unscientific operations of the understanding; and is recognized as an express principle in the earliest speculative essays of man. But here also, as in the case of the mechanical sciences, the development of the idea, and the establishment of the scientific truths which depend upon it, was the business of a succeeding period, and was only executed by means of long and laborious researches, conducted with a constant reference to experiment and observation. Among the most prominent manifestations of the influence of the idea of a medium of which we have now to speak, is the distinction of the qualities of bodies into _primary_, and _secondary_ qualities. This distinction has {294} been constantly spoken of in modern times: yet it has often been a subject of discussion among metaphysicians whether there be really such a distinction, and what the true difference is. Locke states it thus[1\4]: Original or Primary qualities of bodies are 'such as are utterly inseparable from the body in what estate soever it may be,--such as sense constantly finds in every particle of matter which has bulk enough to be perceived, and the mind finds inseparable from every particle of matter, though less than to make itself singly perceived by our senses:' and he enumerates them as solidity, extension, figure, motion or rest, and number. Secondary qualities, on the other hand, are such 'which in truth are nothing in the objects themselves, but powers to produce various sensations in us by their primary qualities, _i.e._ by the bulk, figure, texture, and motion of their insensible parts, as colours, sounds, tastes, &c.' [Note 1\4: _Essay_, b. ii. ch. viii. s. 9, 10.] Dr. Reid[2\4], reconsidering this subject, puts the difference in another way. There is, he says, a real foundation for the distinction of Primary and Secondary qualities, and it is this: 'That our senses give us a direct and distinct notion of the primary qualities, and inform us what they are in themselves; but of the secondary qualities, our senses give us only a relative and obscure notion. They inform us only that they are qualities that affect us in a certain manner, that is, produce in us a certain sensation; but as to what they are in themselves, our senses leave us in the dark.' [Note 2\4: _Essays_, b. ii. c. xvii.]