The fourth sort, which express certainty as to realities of existence, refer to any of three realities.

For every man is able to perceive with absolute certainty that he himself exists, that God must exist, and that finite beings other than himself exist;--the first of these perceptions being awakened by all our ideas, the second as the consequence of perception of the first, and the last in the reception of our simple ideas of sense (chh.i.Section 7; ii.

Section 14; iii.

Section 21; iv, ix-xi).

Agreement of the third sort, of necessary coexistence of simple ideas as qualities and powers in particular substances, with which all physical inquiry is concerned, lies beyond human Knowledge; for here the nominal and real essences are not coincident: general propositions of this sort are determined by analogies of experience, in judgments that are more or less probable: intellectually necessary science of nature presupposes Omniscience; man's interpretations of nature have to turn upon presumptions of Probability (chh.iii.Sections 9-17; iv.