PROP. V.————
"heat" and "colour" express the subjective affections which we call by these names; and they also express certain occult material causes which are supposed to excite them. When we speak of heat in our hand, we mean something very different from what we mean when we talk of heat in the fire. In the one case we mean a sensation; in the other case we mean some inferred property in the fire which occasions that sensation. And so in regard to the other secondary qualities. The words which express them are generally ambiguous, and it is only from the context, or from the relation in which they are spoken, that we are able to determine in which of the two senses (objective or subjective) the terms are employed. In this respect the secondary qualities are said to differ from the primary. But the important circumstance, in the estimation of psychology, and to which our attention is directed in considering this distinction, is, that we have no distinct and assured knowledge of the secondary qualities as they are in themselves, inasmuch as they must be, in their own nature, very different from the sensations to which they give rise. The sensations are all that we are cognisant of: and thus our knowledge of material things, and even the evidence of their existence, would be extremely imperfect, doubtful, and confused, had we no other sources of information respecting them than the subjective affections which their occult qualities are supposed to induce, and no
