The closing chapters of Book IV of the Essay are devoted to a consideration of that kind of apprehension of reality which Locke calls "judgment," as distinguished from "knowledge." "The faculty which God has given man to supply the want of clear and certain knowledge, in cases where that cannot be hand, is judgment: whereby the mind takes its ideas to agree or disagree; or, which is the same, any proposition to be true or false, without perceiving a demonstrative evidence in the proofs" (Bk 4:19:1-2). So-called "scientific" truths being generally of this kind, one would have expected Locke to give here some account of the procedure of inductive science, some directions for the careful and methodical study of the facts, and cautions against the temptations to hasty and unwarranted generalization, such as we find in Bacon's Novum Organum. But instead of this, he contents himself with general observations on the degrees of assent, on reason (and syllogism), on faith and reason, on "enthusiasm," and on wrong assent, or error. The treatment of, that is to say, is limited to general considerations regarding the function of faith and the relations of faith and reason as guides of the human mind.
What is especially significant here is Locke's refusal to oppose faith and reason in the fashion of Bacon and Hobbes, and his refusal to accept any authority which cannot vindicate itself through reason. Even in his insistence upon the necessity of supplementing our knowledge by faith, Locke emphasized the use of reason:
Faith is nothing but a firm assent of the mind: which, if it be regulated, as is our duty, cannot be afforded to anything but upon good reason; and so cannot be opposite to it. He that believes without having any reason for believing, may be in love with his own fancies; but neither seeks truth as he ought, nor pays the obedience due to his maker.... (Bk. 4:27:24)
Locke is at one with the rationalist theologians of his century in their antagonism to an "enthusiasm" which would substitute for the insight of reason and of rational faith, the so called "revelation" of private experience. Against such a view, he insists upon the necessity of judging revelation by reason: "God when he makes the prophet does not unmake the man. He leaves all his faculties in the natural state, to enable him to judge of his inspirations, whether they be of divine original or no.... Reason must be our last judge and guide in everything" (Bk. 4:19:14).
Yet reason clearly limits the field of its own insight; it is only reasonable to believe where we cannot know and yet must act. However, as morality and religion cannot be compassed by reason, such knowledge must be supplemented by faith if we are to fulfill our divine destiny. This is the point of view, not only of the closing chapters of the Essay, but of his Resonableness of Christianity (1695). The aim of this treatise is to recall men from the contentions of the theological schools to the simplicity of the gospel as the rule of human life.:
The writers and wranglers in religion fill it with niceties, and dress it up with notions, which they make necessary and fundamental parts of it; as if there were no way into the church, but through the academy or lyceum. The greatest part of mankind have not leisure for learning and logic, and superfine distinctions of the schools.
What people need is not intellectual insight or theological dogma, but practical guidance. Locke seems less confident than he was in the Essay of the possibility of a rational science of morals. "It should seem, by the little that has hitherto been done in it, that it is too hard a task for unassisted reason to establish morality, in all its parts, upon its true foundation, with a clear and convincing light.... It is plain, in fact, that human reason unassisted failed men in its great and proper business of morality."