FROM DESCARTES TO LEIBNIZ Descartes like Bacon and others before him, was dissatisfied with the state of knowledge in his time. Mathematics was the only study that seemed to be well founded. And he thought that the difference was due to a difference in method. Other studies then followed the scholastic method of citing au thorities for and against certain views. With mathematics it was different. It did not depend on authorities, but set out from certain clear ideas (axioms, postulates, definitions) and then pro ceeded deductively from them to results which were not challenged and contradicted constantly as were the results reached by differ ent thinkers in other fields of study. Descartes resolved, accord ingly, to introduce something essentially like the mathematical method into philosophy. The first requisite was a sure starting point, an Archimedean fulcrum, as Descartes called it. In order to discover it he adopted from Augustine the instrument of "methodical doubt" rejecting everything was open to doubt until he could discover something indubitable. Like Augustine he found that though everything else could be doubted, the reality of the doubt itself could not. "It is easy to suppose that there is no God, no heaven, no bodies, and that we have no hands, no feet, no body; but we can not in the same way conceive that we who doubt these things are not; for there is a contradiction in thinking that that which thinks does not exist when it thinks. Hence the conclusion I think, therefore I am is the first and most certain of all that occurs to one who philosophizes in an orderly way" (Principles of Philosophy, I., vii.). This conclusion, how ever, is only accepted because it is "clear and distinct." Hence the general rule that "whatever I apprehend very clearly and dis tinctly is true" (Meditation III.).
Among such very clear and distinct ideas he includes that of God, the axioms of geometry and such already familiar "eternal truths" as ex nihilo nihil fit, etc., which he also calls innate ideas, in the sense that they are not derived from experience, but are evolved, in due course, by the immanent power of thought itself. Still, Descartes finds it necessary to prove the existence of God, and his main arguments are borrowed from Anselm and Cam panella. Next, relying on the veracity of God, he accepts the reality of the things of the world of experience, with the reserva tion that error arises when we believe what is not very clear and distinct. Such error is possible because belief or judgment is an act of will according to Descartes, who with Duns Scotus believed in the primacy of the will. As to the nature of real things, God is the only self-existent being (or substance in the strict sense) on whom all other things depend for their creation and continu ance. Of created things there are two main types, namely, "ex tended things," or bodies, and "thinking things" or minds. These are created substances, or substances in a secondary sense. Like Augustine Descartes regarded the material and the mental as so utterly different and opposite that there can be no interaction between them. The union of body and soul in man is simply a miracle, whereby the soul (with the help of God—concursus Dei) can direct the motion of the body, but not add to it in any way. When discussing problems which do not directly concern God or man, Descartes shows a much more scientific spirit, though his scientific contributions were of little value except in relation to geometry, to which he rendered services of first-rate importance.
Like Galileo, his senior contemporary, Descartes attempted to explain all natural phenomena mechanically and consequently denied the objective reality of secondary qualities. The lower animals he treated as mere automata or machines; and by attributing an imaginary function to the pineal gland and resorting to the antiquated "animal spirits" of Galen, he tried to introduce at least something mechanical into the relation between the body and the soul. This was not consistent with his conception of the opposed natures of bodies and minds. His followers, accordingly, tried another explanation, known as "occasionalism." According
to this view, body and soul do not influence each other at all, but any change in either is an occasion for divine intervention in the other, so as to produce a corresponding change. This, of course, is supernaturalism pure and simple ; and that is what Cartesian philosophy is essentially. The striking difference be tween Descartes' leaning toward mechanism in science and super naturalism in philosophy is readily intelligible as an instance of that distinction between the two spheres of knowledge, the natural and the supernatural, which was a familiar Catholic tradition traceable to Albertus Magnus and Thomas Aquinas. (See DES CARTES ; CARTESIANS ; KNOWLEDGE, THEORY OF; PSYCHOLOGY, HISTORY OF.) Hobbes (1588-1679) shared Descartes' respect for mathe matics and for deduction, as against the empiricism of Bacon. But, unlike Descartes, he was opposed to supernaturalism, and whereas Descartes confined mechanical explanations within the limits of material nature, Hobbes applied it everywhere, to the mental as well as to the material. The result was a materialistic philosophy. Matter and motion are the only ultimate realities, and explain everything, even knowledge. For all knowledge is derived from sense. "There is no conception in a man's mind, which hath not at first, totally or by parts, been begotten upon the organs of sense." And the sense experiences are produced by the several motions of the matter pressing on the sense organs. "Neither in us that are pressed are they anything else but divers motions ; for motion produceth nothing but motion" (Leviathan, I., i.). For Hobbes mind is matter, and thought, like every process or change, is motion—"all mutation consists in motion." And the same fundamental tendency characterizes all beings, whether human or not, namely, the tendency to persist in their present con dition, whether of motion or of rest. Hobbes does not deny the existence of God. The search for the cause of an effect, and for the cause of that cause, and so on, leads to the thought of an ultimate eternal cause or God ; but we "cannot have any idea of him." The chief work of Hobbes belongs mainly to the history of ethics and of political philosophy. (See HOBBES ; MATERIALISM; ETHICS, HISTORY OF.) Spinoza (1632-1677) marks the culmination of the various tendencies of the Renaissance. He vindicated the autonomy of reason against every kind of authority, subordinating even the Scriptures to it. Rationalism expresses itself most character istically in the attempt to connect things in continuous series or systems of events which suffer no arbitrary incursions from out side in the form of supernatural interventions. The great achieve ments of that classic age of science were essentially expressions of the rationalism of the 17th century. Yet the rationalism of all the great scientists of the time was fragmentary, one-sided; it was a week-day rationalism handicapped with a Sunday irration alism. Spinoza was the complete rationalist, the prince of ration alists. He attempted to interconnect the whole of reality in one organic cosmos, which suffered no cleavage into a natural and a supernatural realm, or into a work-day and a Sabbath vista. For Spinoza, the world is a real universe. It is also a real cosmos, orderly through and through, subject to no arbitrariness, not even the arbitrariness of God. The rejection of the supernatural and the arbitrary constitutes the naturalism of Spinoza. But his conception of the universe avoided sacrificing any one part of it to any other, of the material to the mental, or vice versa. While repudiating any reality outside the cosmos (or Nature) he was careful to include within it whatever could claim reality—not only the material and the mental, but also the divine. They all have their place in the cosmic system. In fact God is the cosmic system. For God is conceived as the Perfect Self-existent ; and that is just what the cosmos or Nature is—hence Deus sive Natura. The All is God, and God is All. This constitutes the pantheism of Spinoza.