The illustrations of the laboratory and the expert remind us of another attitude of thought in which investigation attacks matters hitherto reserved. The growth, for example, of freedom of thought during the Renaissance was a revelation of the intrinsic momentum of the thought-process itself. It was not a mere reaction from and against mediaeval scholasticism. It was the continued operation of the machinery which the scholastics had set a-going. Doubt and inquiry were extended into the region of particulars, of matters of fact, with the view of reconstituting them through discovery of their own structure, no longer with the intention of leaving that unchanged while transforming their claim to credence by connecting them with some authoritative principles. Thought no longer found satisfaction in appraising them in a scale of values according to their nearness to, or remoteness from, fixed truths. Such work had been done to a nicety, and it was futile to repeat it. Thinking must find a new outlet. It was out of employment, and set to discover new lands. Galileo and Copernicus were travelers--as much so as the crusader, Marco Polo, and Columbus. Hence the fourth stage--covering what is popularly known as inductive and empirical science. Thought takes the form of inference instead of proof. Proof, as we have already seen, is accepting or rejecting a given proposition on the ground of its connection or lack of connection with some other proposition conceded or established. But inference does not terminate in any given proposition; it is after precisely those not given. It wants more facts, different facts. Thinking in the mode of inference insists upon terminating in an intellectual advance, in a consciousness of truths hitherto escaping us. Our thinking must not now "pass" certain propositions after challenging them, must not admit them because they exhibit certain credentials, showing a right to be received into the upper circle of intellectual society. Thinking endeavors to compel things as they present themselves, to yield up something hitherto obscure or concealed. This advance and extension of knowledge through thinking seems to be well designated by the term "inference." It does not certify what is otherwise doubtful, but "goes from the known to the unknown." It aims at pushing out the frontiers of knowledge, not at marking those already attained with signposts. Its technique is not a scheme for assigning status to beliefs already possessed, but is a method for making friends with facts and ideas hitherto alien. Inference reaches out, fills in gaps. Its work is measured not by the patents of standing it issues, but by the material increments of knowledge it yields. _Inventio_ is more important than _judicium_, discovery than "proof."