OF KNOWLEDGE AND PROBABILITY SYNOPSIS OF THE FOURTH BOOK.
Locke's review of the different sorts of ideas, or appearances of what exists, that can be entertained in a human understanding, and of their relations to words, leads, in the Fourth Book, to an investigation of the extent and validity of the Knowledge that our ideas bring within our reach; and into the nature of faith in Probability, by which assent is extended beyond Knowledge, for the conduct of life. He finds (ch. i, ii) that Knowledge is either an intuitive, a demonstrative, or a sensuous perception of absolute certainty, in regard to one or other of four sorts of agreement or disagreement on the part of ideas: — (1) of each idea with itself, as identical, and different from every other; (2) in their abstract relations to one another; (3) in their necessary connexions, as qualities and powers coexisting in concrete substances; and (4) as revelations to us of the final realities of existence. The unconditional certainty that constitutes Knowledge is perceptible by man only in regard to the first, second, and fourth of these four sorts: in all general propositions only in regard to the first and second; that is to say, in identical propositions, and in those which express abstract relations of simple or mixed modes, in which nominal and real essences coincide, e. g. propositions in pure mathematics and abstract morality (chh. iii, v-viii). 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. SectionS 11-17; vi, xiv-xvi). In forming their stock of Certainties and Probabilities men employ the faculty of reason, faith in divine revelation, and enthusiasm (chh. xvii-xix); much misled by the last, as well as by other causes of 'wrong assent' (ch. xx), when they are at work in 'the three great provinces of the intellectual world' (ch. xxi), concerned respectively with (1) 'things as knowable' (physica); (2) 'actions as they depend on us in order to happiness' (practica); and (3) methods for interpreting the signs of what is, and of what ought to be, that are presented in our ideas and words (logica).