The Nature of Rationalism. Although the new science grew apace, it was not altogether a safe vocation. Natural science involves metaphysical questions at every point. The scientist at this time, therefore, found himself often in delicate relations with the jealous church guardians. A scientific explanation of the universe might antagonize the church dogma concerning God, creation, and the final outcome of the world. The church doctrine concerning the soul, too, its nature and its immortality, its relation to the body, might be antagonized by physiological and psychological discussions. In such dilemmas as these the natural scientist was not successful in pretending to isolate himself entirely from theology and in assuming an attitude of aloofness to it. Galileo might declare that, whatever the results of his investigations in physics might be, they had nothing to do with the Bible; but he sorrowfully found that the Inquisition thought otherwise. Copernicus found that his astronomical theories came into conflict with church dogma, and he was tormented by his bishop. Kepler spent his later years in a deadly struggle with both Protestantism and Catholicism. Bacon and Hobbes lived in a country where their personal safety was fairly secure, nevertheless Bacon disguised his position by using large words and Hobbes was untroubled because he accepted the religion of his sovereign.
If the position of those was difficult who tried to keep themselves strictly within the limits of science, how much more fraught with personal danger was the position of those who openly constructed a new metaphysics? It would mean that a challenge was issued to the old Scholasticism by the same human reason that had already challenged and overthrown the old science. The group of men who did this were the Rationalists. The Rationalists were interested in science, but they were more interested in the metaphysical problems that science aroused. The human reason had been successful in the reconstruction of physics by the use of mathematics. Why should it not also be able to reconstruct metaphysics and set it, too, upon a mathematical basis? The leaders of this school were Descartes, Spinoza, Leibnitz, and the Occasionalists,—Malebranche and Geulincx. The Rationalists advanced a new conception not only of nature, but of God; new theories not only of the human body, but of the soul. Their task was the dangerous one of bravely invading the hitherto impregnable realms of the spirit.
The task of the Rationalists was rendered the more difficult because, for the first time in the history of European thought, the inner and outer worlds had been completely sundered. For the first time do we meet with a clear-cut and positive dualism. The history of the growth of this dualism had been a long one, and to it the Greek Sophist, the Stoic, and the Christian had each contributed his share. However, Galileo and his fellow scientists in this period of the Renaissance had so reconstructed the old “world of nature” that it had become irreconcilable to the “world of grace.” These scientists believed that nature must be made to explain itself; its events must be conceived as necessitated; its processes as having the inevitableness of a machine. From the revolutions of the planets to the circulation of the blood, the movements of nature can be measured. The law of nature, that is conceived to underlie all this science, is mechanical causation. The researches of the scientists of the Renaissance had yielded a rich world of brute, inevitable, and scientific facts, and these stood in absolute fundamental contrast to the world of spiritual facts which were embodied in the church dogma. Apparently the problem of reconciling the “world of nature” and the “world of grace” had been solved by St. Thomas Aquinas in mediæval times. Now, however, the “world of nature” had been so reconstructed that the question was re-opened. How is the new “world of nature” to be brought into harmonious relation with that old, persistent, and settled dogma of the church? How can the newly conceived mechanism of nature be harmonized with the realm of free conscious spirits, without giving up the conception of God as a rational being, and also without depriving the soul of its power of initiation? The new science had therefore made it especially difficult on the one hand to reconcile a mechanical universe with an omnipotent God, and on the other to reconcile the mechanical human body with the free soul.
The struggle of the Renaissance with the Middle Ages is therefore concentrated in the development of the doctrine of this Rationalist School. It is studied here even better than by reading the two periods side by side. In Rationalism the Scholasticism of the Middle Ages and the Science of the Renaissance meet. Rationalism was a new science, but it was a new theology as well. It was a new scholastic philosophy; for, while the Rationalists thought that they were giving the death blow to mediæval philosophy, they were instead only replacing it with another scholasticism. In their attempt, by means of the mechanical theory, to get an absolute system of knowledge upon which thought can rest, the Rationalists were acting in the spirit of the schoolmen. In fact, no schoolman ever showed more vigor or more dogmatic confidence in his philosophy. To the mathematical eye of the Rationalist there was absolutely nothing mysterious in the physical universe or in the spiritual realm. All things in heaven and earth could be made clear. The declaration of the Rationalists was the call of freedom, but it was as hazardous as it was ambitious; and the church with its assured revelations always stood opposed to the realization of freedom. So we shall find Descartes spending his whole life trying to trim his sails that he may not offend the Inquisition; Spinoza saving himself from both the Jews and the Christians by living in obscurity and publishing nothing; Leibnitz constructing philosophy with the avowed purpose of reconciling science and religion.
The Mental Conflict in Descartes. The strife between the spirit of the Middle Ages and that of the Renaissance appears in Descartes more strikingly than in any other thinker of this time. He shows, on the one hand, all the conservatism of a churchman of mediæval time in his respect for institutional authority; on the other hand, his intellectual activity places him among the leading scientists of the Renaissance. In no other thinker does the conflict between the Old and the New appear so unsettling; in none does the antagonism between the scholastic world of spiritual things and the mechanical world of science appear so irreconcilable. He suffered a life-long mental strife, for within himself mediævalism and science were engaged in an unending dramatic struggle. The philosophy of Descartes was a compromise between his traditions and his scientific genius; and his philosophy never overcame his conflicting motives. The admirers of Descartes have called him the father of modern thought, and this is partly true. The father of the modern scientific method was Galileo. Descartes, on the other hand, pointed out the incontestable principle from which modern thought has proceeded; he won his place in the history of philosophy by attempting to harmonize the old scholasticism with the new science under this single principle.
The Life and Philosophical Writings of Descartes (1596–1650).26
(1) As Child and Student (1596–1613).
At home until he was eight years old (1596–1604).
At the Jesuit school at La Flèche until he was seventeen (1604–1613).
(2) As Traveler (1613–1628). Descartes studies “the book of the world.”
At Paris (1613–1617), in retirement and study.
In Holland (1617–1619), nominally attached to the army of Maurice.
First Journey (1619–1621), going through Bavaria, Austria, north to the shores of the Baltic and back to Holland. The greater part of these two years were spent in Bohemia, enrolled in the army of the Emperor. He was on this journey when his mental crisis occurred,—at Neuberg, in Austria, in 1619. It was then that he discovered either analytical geometry or the fundamental principle of his philosophy.
In Paris again, 1623.
Second Journey (1623–1625), to Switzerland and Italy, making a pilgrimage to the shrine of Loretto.
(3) As Writer (1629–1650).
In Holland (1629–1649). For the sake of absolute seclusion from inquisitive visitors, Descartes changed his residence in Holland twenty-four times and lived in thirteen places. All his correspondence passed through Mersenne. During these twenty years he made three journeys to France. Thus this period of absolute retirement became his period of literary production, chiefly between the years 1635 and 1644. He wrote his
Le Monde (1630–1632), published posthumously.
(4) In Stockholm, Sweden (1649–1650). The romantic side of the life of Descartes appears in his book on the Passions, which he wrote for the Princess Elizabeth, and also in his acceptance of the invitation of the Queen of Sweden to reside at her court and become her tutor. He died there from the rigors of the climate after a residence of one year.
The Two Conflicting Influences upon the Thought of Descartes. On the one hand, all the ties of inheritance, family influence, and early education allied Descartes with the spirit of the Middle Ages. A delicate constitution made him shrink from public controversy and the public eye. He even made a half apology for his pursuit of science by saying that he was seeking to reform his own life, and that it was absurd for an individual to attempt to reform a state. His family on both sides belonged to the landed gentry, and he was therefore bound by caste to the support of institutional authority. He was educated in the Jesuit school of La Flèche, and this most conservative of ecclesiastical influences restrained him from following the logical conclusions of his own thought. He was therefore both physically timid and intellectually aloof. In 1632 he was about to publish Le Monde, which was a scientific description of the origin and nature of the universe, and agrees in part with the Copernican theory. It was a treatise which would naturally conflict with the teaching of the church. He learned of the trial of Galileo at Rome, and he never dared to publish the book.
The rival spirit speaking in Descartes was the new scientific spirit of the Renaissance. He had a genius for mathematics even when he was at school at La Flèche. On his going to Paris he became the centre of the most notable scientific circle in France—a circle composed of such men as the Abbé Claude Picot, the physician Villebressieux, the optician Ferrier, the mathematician Mersenne, and many other scientists and theologians. But he became dissatisfied and made some long journeys in order to study “the book of the world.” His discovery of his method and his philosophical principle was the result. In mathematics he was the discoverer of analytical geometry and was the first to represent powers by exponents; in physics he stated the principle of the refraction of light in trigonometrical form; he explained the rainbow; he weighed the air. The same industrious application of the new scientific methods that yielded great results in science, also resulted in his development of his philosophy. Love for original discovery made Descartes disdainful of all scientific authorities and even contemptuous of his notable contemporaries, Galileo and Harvey. He mentions by name Plato, Aristotle, Epicurus, Campanella, Telesio, and Bruno, but he claimed that he learned nothing from any one except Kepler. He felt himself to be above criticism, and in his self-arrogating dogmatism he is the type of the modern practical individualist. He defined truth as candor to one’s self, and both in his practical life and in his theoretical ideal there is an entire absence of utilitarianism.
The Method of Descartes. Both science and scholasticism show themselves in the method of Descartes. He attempted to construct a philosophical method entirely in the scientific spirit of the Renaissance, but in the application of it he showed his scholastic training. Surfeited with inadequate and traditional methods he felt the need of some single principle by which all knowledge might be systematized, and he was sure that mathematics would furnish the key. Rational science was to Descartes only mathematics. Truth is to be found not in metaphysics, nor in empirical science. Descartes’ philosophical aim was to establish a universal mathematics. Descartes was not entirely faithful to Galileo’s mathematical principle in his employment of it, and his influence in metaphysics was thereby all the greater; for in the development of his method he found assistance in the traditional scholastic methods. Descartes was original in insisting upon finding the existence of an absolute and undeniable principle before any progress could be made. Such an absolute principle can be obtained only by an inductive sifting of all ideas. From this all further truths must be obtained by deduction. Every true philosophy must therefore be an induction or analysis of ideas, and secondly, a deduction or synthesis. The great contribution of Descartes was therefore this: to the inductive method of Bacon and the deductive method of Galileo, he added an absolute principle which must be taken as the basis of both induction and deduction.27
Induction—Provisional Doubt—The Ultimate Certainty of Consciousness. The philosophical proclamation of Descartes was characteristically French, for he demanded the same return to an uncorrupted nature for the understanding that Rousseau many years later demanded for the heart. The first step of Descartes was also French in its demand for absolute clearness, which from his youth had shown him to be so passionately fond of mathematics. The way to such clearness is through provisional doubt. Let us purify the understanding by delivering it of the rubbish of traditional opinions, taken upon the say-so of others. By this negative induction of received knowledge, let us see if there is anything positive and certain. In Descartes’s Meditations, in “a dramatic dialogue with himself,” he portrays his own intellectual struggle to gain uncontaminated truth. He makes an induction of all kinds of knowledge and challenges each as it appears. Nothing is to be accepted as true until it has proved itself true. All facts are subjected to rigid scrutiny. Descartes doubts the testimony of the senses, the existence of the material world, the existence of God. But this induction is provisional, even if it is radical. While none of the usually accepted truths are found by him to be undeniable and absolute, yet Descartes has an ulterior purpose in challenging them. Greek skepticism had no further end than doubt, while at the other extreme Anselm and the orthodox scholastics had refused to doubt at all. The method of Descartes is contrasted both with that of Anselm and with that of the Skeptics, for he doubts in order that he may know. Dubito ut intelligam. Doubt is necessary, but only as a means to an end; and that end is knowledge. Descartes proclaimed for the modern individual the privilege and the duty of rationalizing his own beliefs.
In such an inductive sifting of traditional beliefs, are there any that can be called knowledge? Is there one whose reliability cannot be successfully doubted? Not a single one, except the thinking process itself. I am certain that I am conscious. Even when in my universal doubt I say that nothing is certain, I am at least certain that I doubt. I am, therefore, contradicting my universal skepticism. To doubt is to think; in doubting, consciousness is asserting its existence. Skepticism is self-contradictory. An induction of our ideas reveals at least this one absolutely certain principle: I, as thinking, am. Cogito ergo sum. My own existence is an intuitive truth that accompanies every state of mind. This is the best known portion of Descartes’s philosophy, and perhaps it is in part to the Latin formula of it that it owes its widespread acceptance. It is criticised as trifling, even if it be true; and as reasoning in a circle. Yet it must be remembered that Descartes does not intend the ergo sum (“therefore I am”) to be a conclusion of a syllogism of which Cogito (“I think”) is the minor premise. This formula is not an inference, but an intuition, which is revealed by induction as the certain background of all knowledge.
Three things are to be learned from this fundamental principle, said Descartes: (1) The first is that man has gained a criterion of truth. The characteristic of this principle that makes it reliable and certain is its clearness and distinctness. Clearness and distinctness of ideas is the proof of their truth. All true ideas will therefore have the mathematical and intuitive certainty that the idea of the existence of the self has. (2) The second lesson from this fundamental principle is that the existence of the soul is more certain than that of the body. The soul is more important and independent than the body. This is the subjective point of view of modern times. The modern man views the world as the representation or the creation of his thinking soul. (3) The third lesson from this principle concerns the nature of the soul. How long do you exist? As long as you think. (Sum cogitans.) True existence is rational thinking, and God alone has it. Feelings and passions are obscure ideas.
Deduction—The Implications of Consciousness. For Descartes reality lies within the Self; and the next question before him is how to get out of the Self. Knowledge that is confined to the Self and its states is called, technically, solipsism. Such knowledge amounts to little; indeed, it is not knowledge at all. Certainty of self-existence is the minimum amount of knowledge—merely the starting point of knowledge. Descartes proposes to escape from this solipsism by the use of logic. His method from this point on is ostensibly deductive, although he introduces by the side door other ideas than the idea of Self to make his proof complete. Descartes maintains that any idea will be as true as the consciousness that accompanies it, just as a proposition in geometry partakes of the truth of the axioms from which it is derived. Now my consciousness contains many ideas; some of them seem to be the product of my imagination; some seem to be adventitious; some are innate. It is upon the innate ideas that Descartes depends to get him out of his solipsism, for they are not created by the Self and they have the qualities of truth—a conscious clearness and distinctness. Among these innate ideas is the idea of God as a perfect being.
The Existence of God.28 As a deduction from consciousness, the idea of God would prove to be a very useful one to Descartes, provided it had reality. For it is evident that consciousness can testify only to the existence of itself and its own states. How do I know the reality of anything else? Am I confined within the circle of my own thinking? Is all that I can say of this or that, “It is real to me”? Are all things only the phantasmagoria of my own brain, testifying only to the existence of myself? Descartes thought that the idea of God relieved him of this solipsism. If he could demonstrate God’s existence, he would then be able to demonstrate the existence of the material universe. The problem was so highly important to Descartes that he threw it into several different arguments. The complications with which these arguments are filled must be passed over here, and the arguments stated in their simplest forms.
(a) Two are ontological arguments, that is, arguments from the character of the conception of God’s nature.
(1) A Simple Deduction. If I have in my consciousness any idea as clear and distinct as my idea of Myself, it must have existence like Myself. My idea of God has just that clearness and distinctness; and therefore God exists.
(2) The Geometrical Argument, so called by Descartes. Some ideas have properties so immutable that, when we think the ideas, we necessarily think their properties. Such is the idea of a triangle; when I think of a triangle, I must think of it as having its three angles equal to two right angles. Such is also my idea of God; I must think of him as perfect and existing. He would not be God, i. e. a perfect Being, if He did not exist.
The reader will recognize this as a re-statement of the argument by St. Anselm. As such it raised a tempest of controversy in Descartes’ time, and was attacked from all sides.
(b) Two are causal arguments, that is, based on the assumption of the equality of cause and effect. Only one of these arguments will be cited here. This is known as
The Cartesian Argument. I have an idea of a perfect Being. This idea must have an adequate cause. Therefore God must exist, for only He, and no imperfect being, can be the adequate cause of my idea of perfection.
The ontological arguments given by Descartes are evidently deductions from the certainty of self-consciousness. The question which we immediately raise concerning them is, Are they true? As to the causal arguments, Descartes is breaking away from his original assumption, viz., that self-consciousness is the only certainty, and is introducing another assumption, viz., the certainty of the law of cause. The question, then, that the thoughtful student asks, is, Does Descartes really escape from his solipsism?
The Reality of Matter. It will be seen that Descartes is trying to deduce from the certainty of the idea of self-consciousness the certainty of other ideas, as propositions are deduced in geometry from axioms. The existence of God is an implication of human consciousness. Now Descartes points out that the existence of matter is implied in the existence of God. Descartes is interested in material science, and it is important for him to prove the reality of matter. Here again his scholastic training comes into play. Since God has all the attributes of a perfect being, He must be veracious. If there were no God, but only a deceiving Devil, the external world might be only a fiction, created to deceive us. But God exists, and we can trust that He would not continually deceive men about the existence of nature. An atheist could have no science, but to Descartes,
“God’s in His heaven—
All’s right with the world.”
Of course, man is constantly in error about the character of physical things, but these errors arise from his misinterpretation of them. Nature in some form lies before man, or else God in His truthfulness does not exist. The essence of matter is extension (see below), and whatever my interpretation of it, something extended lies before me to be interpreted.
This is the skeleton upon which Descartes constructs his theory. Even this cursory examination of it shows the obvious attempt to explain “the world of grace” by the method of mathematics, and it is quite consistent with the spirit of the Renaissance. The existence of God and the existence of matter are deduced in turn from the axiom of all thought, the Self; while matter is further described as the extended or the measurable. Thus Descartes has tried to construct a bridge between the scholastic concepts and the science of the Renaissance. The three realities, the Self, God, and matter, which Descartes often speaks of as intuitively certain, have obviously a differing cogency. The reality of consciousness is the ground from which the other two are derived. In asserting its primacy, he is voicing the spirit of the Renaissance even more clearly than did Galileo and Bacon. For Descartes in this has gone back of the objective facts to a single subjective principle; whereas the deductive principles of Galileo were objective. In this respect Descartes is the founder of the subjective method of modern thought, and in identifying the Self as the reason he became the founder of rationalism. In any case he established a background for epistemology, or the theory of knowledge. But in his derivation of the other two realities—God and matter—he shows how persistent was the scholastic current in his thought. Although he declared them to be intuitively known, they evidently are not so in the same sense that self-consciousness is; and he felt obliged to support them by traditional scholastic arguments.
God and the World. Leaving these fundamental principles of Descartes, we now come to a consideration of a few of the details of his philosophy. Descartes’ world is a dualism in which conscious being stands in contrast with space objects. God is related to the world of mind on the one hand and to the world of matter on the other. The order in which Descartes came upon the three substances—the Self, God, and matter—is, however, not the order of their reality. In reality God is the primary substance, for He depends only upon Himself. Matter and the Self are relative or created substances, for they depend upon God. Matter and mind have different modes of appearing: the modes of matter are form, size, position, and motion. The modes of mind are ideas, judgments, and will. Thus mind is so essentially different from matter, as can be seen in their respective modes, that God stands in a different relation to each.
The Relation of God to Matter. Descartes here investigates the realm in which he has the deepest interest; but he makes a concession at the very beginning. He divests things of their qualities and finds the essence of matter to be extension. Qualities are not resident in things, but are the result of our sensations. Sense-perception is knowledge of qualities, and therefore obscure knowledge; while clear or intellectual knowledge is of quantities. But there is one quality common to matter,—extension. Space, extension, and matter are the same. There is no space that is empty, no matter that is not extended. An extended or material body has, however, in itself no principle of motion. It cannot move itself. It must be moved by an external cause, and the whole universe must be a mechanism whose movements have their first cause in God. Matter in its modes of motion and rest has God as its first cause or unmoved mover; and under matter is included everything extended,—inanimate objects, the lower animals, and the bodies of men. To this world of matter God stands in the relation of an inventor to his machine.
The Relation of God to Minds. The essential nature of minds is thought. Mind is therefore different from matter because it is unextended and free. The two relative substances have nothing in common except that they are related to God. The relation of God to minds is, however, very different from His relation to matter. God is not the unmoved mover of minds, but He is the perfect and infinite mind to which our finite minds turn as their ideal. God thinks and wills perfectly what we think and will imperfectly. He is not the mechanical but the teleological cause of minds, their ens perfectissimum, the goal of all mental aspiration.
The Relation of Mind and Body. In proportion as Descartes clearly defined mind and body, and referred each back to its own principle, the impossibility of connecting the two became apparent. Descartes intended that his theory should, above everything else, clear philosophy of all obscurities. So he divided the world into two relative substances,—mind and matter,—each operating in its own realm, each exclusive of the other. The intention of Descartes is to be a consistent dualist. But there was one point where, with one eye on the church, he had to qualify for ethical considerations his scientific principle of matter. That is the point where the human body acts upon the soul and the soul acts upon the body.
There was little trouble for Descartes in conceiving the movements of inanimate bodies, plants, and all the lower animals as purely mechanical and automatic, with their first cause in God. From his own investigations he felt obliged to regard many of the human functions as automatic also. But his ethical and theological interests compelled him to think of man as exalted above the rest of creation. Theology has always been in a sense aristocratic, and has drawn a line between man and other things. Man alone has a soul in his body. The soul of man is immortal and free, and must therefore have control over the body; nevertheless the soul of man must be conscious of the impressions that come through the body. Here the science of the Renaissance and the scholasticism of the Middle Ages refuse to be reconciled in the philosophy of Descartes. When it became a question between Descartes’ scientific theory of matter operating itself mechanically and the church doctrine of a spiritual will operating the matter of the human body, the scientific theory had to yield. How does Descartes yield gracefully to the theological requirements and bring together the two unlike worlds of matter and mind in the human personality?
Descartes’ explanation of the relation of human mind and body reminds us of the mythical explanations of Paracelsus. The soul is united to all parts of the body, but its point of contact with the body is the pineal gland, and this contact is made possible through the animal spirits (spiritus animales) or the fire atoms in the blood, a revived Greek conception. The pineal gland is a ganglion in the centre of the brain, which biologists tell us is a defunct eye, but which Descartes conceived to be the seat of the soul. Descartes maintained that the animal spirits, having been distilled by the heart, ascend by mechanical laws from the heart to the brain, and then descend to the nerves and muscles. When they pass through the pineal gland, they come in contact with the soul. The soul exercises influence on the body by slightly moving the gland and diverting the animal spirits. In this way the emotions and sensations are to be explained. The movement of the pineal gland by the animal spirits causes sensations in the soul; the movement of the gland by the soul changes the movement of the animal spirits, and is an exhibition of free action. But this does not add to or subtract from the energy. It merely changes the direction of energy.
The Influence of Descartes. Although the philosophy of Descartes was forbidden in the University of Oxford, was proscribed by the Calvinists in Holland, and his works were placed upon the Index by the Catholics, it created a profound impression on the theology, science, and literature of the seventeenth century. It spread over Europe in a somewhat similar way to the Darwinian evolution theory in modern times. Its success was immense, many standard men rallied to its support, and everything before Descartes was considered to be antiquated. Among philosophers his doctrine had an internal development in a natural way along the lines of the problems which he had left unsolved. A philosophical development, the source of which can be traced directly back to Descartes, went on until Kant published his Critique in 1781. This has later been called the School of Rationalism in Germany, France, and Holland. The most important members of this school—the Occasionalists, Spinoza, Leibnitz, and Wolff—we shall consider in their place. Descartes had an important immediate following in the group, who go by the name of Occasionalists; but his most important successor, who can hardly be called his disciple, was Spinoza.
Descartes’ method had a peculiar fate. His followers misunderstood it, exactly reversed it, and obtained very fruitful results. Descartes himself had hoped to see induction employed in most metaphysical problems. He regarded deduction as of use only in proceeding from one self-evident fact to another. But the following Rationalists used the deductive method entirely and tried to systematize ethics after the manner of Euclid. They deduced their systems from some assumed principle. This tendency was first seen in the Port Royal logic, and was completed by Spinoza.
The Relation of the Occasionalists and Spinoza to Descartes. The development of the doctrines of the Occasionalists and Spinoza from Descartes was an attempt to make clear the conception of substance. Since substance was the most important scholastic category, it is easy to see why Spinoza’s teaching became thoroughly scholastic. Descartes had used the term “substance” in a very loose way to apply to God as infinite, and to minds and bodies as finite. He speaks of God as the only substance, and yet of consciousness and bodies as created substances. Such ambiguity must be overcome, if a philosophy which prided itself on making everything “clear and distinct” was to stand. Descartes had fallen short of justifying his attempt to put metaphysics completely upon a mathematical basis, although this had been his original problem. The obscurity of the spiritual world still remained, because Descartes had left the concept of the spiritual substance undefined. The world of the spirit was still an unknown country. The spiritual substance had not been made clear and distinct, and there still remained the ontological problem of the relation between mind and matter, and the psychological problem of the relation between the individual soul and its body.
Descartes had, however, defined clearly the concept of the substance of matter—the substance with which the natural scientist works. He had accomplished this, to be sure, by destroying the essential distinctions between material things. A “thing” is essentially a substance in which many qualities inhere, e. g. a piece of sugar having whiteness, sweetness, etc. Material substances were alike in that all were essentially extension. All else besides extension in any particular finite thing was a modification of extension. A lump of sugar was essentially the same as a lump of salt in that both were extension; the saltness, sweetness, etc., were secondary. Now this makes the nature of bodies very clear; and Descartes proposed to reduce the substance of the states of mind to the same clearness, but he did not do it. He was interested in natural science and he developed his rationalism only with reference to matter. Bodies are parts of space or corpuscles, which are mathematically infinitely divisible, but perceptually are not further divisible. As far as he went, Descartes was clear enough.
The Occasionalists and Spinoza represent the second stage in the development of Rationalism. Both tried by making clear the meaning of spiritual substance to define the relationship of God to the material world. Both tried to state the problem in other words, to overcome the dualism between mind and matter, and to reconstruct the old “world of grace” so that it would be consistent with the new world of science. The Occasionalists, whose chief exponents were Malebranche and Geulincx, we shall dismiss with only a few words, while considerable attention must be given to the teaching of Spinoza. Malebranche tried to do for the mental world what Descartes had done for the world of matter. Since no knowledge is possible except in God, he claimed that the modes of finite minds—our ideas, judgments, imaginations—are alike in essence in being modifications of the universal reason of God. God is so far the “place of minds” as space is the place of bodies. All our ideas participate in God’s reason, and all our volitions are the modifications of the will of the Divine, just as bodies are modifications of extension. What then is the relation, asked Geulincx, between bodily movement and the states of consciousness? Why does my arm move when I wish to move it? By the mediatory power of God. The thought in my mind is the “occasional cause” of the movement of my arm, while God is the true cause of the movement. The movement of the human body is therefore, like the movement of all matter, a continuous miracle caused by an ever watchful Deity, who keeps body and mind in harmony. Spinoza completed his pantheism before Malebranche had prepared the way. He formulated a complete doctrine of substance, conceiving material bodies to be essentially the same in being modes of extension, and mental phenomena to be essentially alike in being modes of thought. But more important was his further teaching that on that account the two series have no relation to each other. That is to say, Spinoza reduced the whole difficulty to clearness and distinctness by reducing the three substances of Descartes to one. For this reason Spinoza was a more complete Rationalist than Descartes; and he was assisted in this construction of a mathematical Rationalism by two facts: he held himself strictly to the deductive method, and he was free from social and ecclesiastical ties. Spinoza is the truest utterance of his time in its effort to make all things clear; and this is not contradicted by the fact that he had little influence in shaping contemporary thought.