unity is indivisibility, whereas everything known to be one is merely undivided or individual, and that there must be simple because there are compound substances, although composition only requires simpler or relatively simple elements. On the other hand, under the influence of the mechanics of his day, which had hardly distinguished between inertia, or the inability of a body to change itself, and resistance or the ability of bodies to oppose one another, he concluded that, as inertia is passive, so is resistance, and refused to recognize that in collision the mutual resistance of moving bodies is a force, or active power, of changing their movements in opposite directions. From these two arbitrary hypotheses about corporeal motion, that it requires indivisibly simple elements, and that it offers only passive resistance, he concluded that behind bodies there must be units, or monads, which would be at once substantial, simple, indivisible and active. He further supposed that the monads are “incorporeal automata,” not interacting like bodies, but each perceiving what was passing in the other, and acting in consequence by appetite, or self-acting. Such mentally endowed substances might be called souls; but, as he distinguished between perception and apperception or consciousness, and considered that perceptions are often unconscious, he preferred to divide monads into unconscious entelechies of inorganic bodies, sentient souls of animals, and rational souls, or spirits, of men; while he further concluded that all these are derivative monads created by God, the monad of monads. All derivative monads, he allowed, are accompanied by bodies, which, however, are composed of other monads dominated by a central monad. Further, he explained the old Cartesian difficulty of the relation of body and mind by transforming the Spinozistic parallelism of extension and thought into a parallelism between the motions of bodies and the perceptions of their monads; motions always proceeding from motions, and perceptions from perceptions; bodies acting according to efficient causes, and souls according to final causes by appetition, and as if one influenced the other without actually doing so. Finally, he explained the concomitance of these two series, as well as that between the perceptions of different monads, by supposing a pre-established harmony ordained by the primitive monad, God.
Up to this point, then, Leibnitz opened one of the chief avenues to metaphysical idealism, the resolution of the material into the immaterial, the analysis of bodies into mental elements. His theory of bodies involved an idealistic analysis neither into bodily atoms nor into mathematical units, but into mentally endowed simple substances. There remained, however, his theory of the nature of bodies; and here he hesitated between two alternatives. According to one alternative, which consistently flowed from the psychological idealism of Descartes, as well as from his own monadism, he suggested that bodies are real phenomena; phenomena, because they are aggregates of monads, which derive their unity only from appearing together to our perceptions; real phenomena well founded, because they result from real monads. In support of this view, he said that bodies are not substances, though substantiata; that their apparent motion and resistance are results of the passions of their monads; that their primary matter is nothing but passive power of their monads; that the series of efficient causes between them is merely phenomenal. According to this alternative, then, there is nothing but mental monads and mental phenomena; and Leibnitz is a metaphysical idealist. According to the other alternative, however, he suggested that at least organic bodies are compound or corporeal substances, which are not phenomena; but something realizing or rather substantializing phenomena, and not mere aggregates of monads, but something substantial beyond their monads, because an organic body, though composed of monads, has a real unity (unio realis). From this point of view he believed that the real unity of a body is a vinculum substantiale, which gives it its real continuity and is the principle of its actions; that its primary matter is its own principle of resistance; and that it has not only this passive, but also an active, power of its own. He suggested that this theory of the substantial unity of a body might explain transubstantiation, by supposing that, while the monads and phenomena of bread remain, the vinculum substantiale of the body of Christ is substituted. He feared also whether we can explain the mystery of the Incarnation, and other things, unless real bonds or unions are added to monads and phenomena. According to this alternative, these organic bodies are compound or corporeal substances, between monads and phenomena; and Leibnitz is a metaphysical realist. He was held to this belief in the substantiality of bodies by his Christianity, by the influence of Aristotle, of scholasticism and of Cartesianism, as well as by his own mechanics. But the strange thing is that at the very end of his life and at the very same time, in 1714–1716, he was writing the idealistic alternative to Remond de Montmort and Dangicourt, and the realistic alternative to Father des Bosses. He must have died in doubt. We cannot, therefore, agree with many recent idealists who regard Leibnitz as one of themselves, though it is true that, when stripped of its realism, his metaphysics easily passed into the metaphysical idealisms of Lotze and of Fechner. It is true, also, that on its idealistic side the philosophy of Leibnitz is the source of many current views of panpsychism, of psychophysical parallelism as well as of the phenomenalism of bodies, and of the analysis of bodies into mental elements.
2. Locke to Hume.—Meanwhile in England, Locke, though differing from Descartes about the origin of ideas, followed him in the illogical combination of psychological idealism with metaphysical realism. He thought that we perceive nothing but ideas both of primary and of secondary qualities, and yet that somehow we are able to infer that, while our ideas of secondary qualities are not, those of primary qualities are, like the real qualities of external things. Berkeley saw the inconsistency of this position, and, in asserting that all we perceive and all we know is nothing but ideas in “mind, spirit, soul, or myself,” has the merit of having made, as Paulsen remarks, “epistemological idealism the basis of metaphysical idealism.” According to him, a body such as the sun is my idea, your idea, ideas of other minds, and always an idea of God’s mind; and when we have sensible ideas of the sun, what causes them to arise in our different minds is no single physical substance, the sun, but the will of God’s spirit. Hume saw that in making all the objects of perception ideas Berkeley had given as little reason for inferring substantial souls as substantial bodies. He therefore concluded that all we know from the data of psychological idealism is impressions or sensations, ideas, and associations of ideas, making us believe without proof in substances and causes, together with “a certain unknown, inexplicable something as the cause of our preceptions.” We have here, in this sceptical idealism, the source of the characteristically English form of idealism still to be read in the writings of Mill and Spencer, and still the starting-point of more recent works, such as Pearson’s Grammar of Science and James’s Principles of Psychology.
3. Kant and Fichte.—Lastly, in Germany, partly influenced by Leibnitz and partly roused by Hume, Kant elaborated his transcendental or critical idealism, which if not, as he thought, the prolegomena to all future metaphysics, is still the starting-point of most metaphysical idealists. Kantism consists of four main positions, which it will be wellKant. to lay out, as follows:—
a. As to the origin of knowledge, Kant’s position is that sense, outer and inner, affected by things in themselves, receives mere sensations or sensible ideas (Vorstellungen) as the matter which sense itself places in the a priori forms of space and time; that thereupon understanding, by means of the synthetic unity of apperception, “I think”—an act of spontaneity beyond sense, in all consciousness one and the same, and combining all my ideas as mine in one universal consciousness—and under a priori categories, or fundamental notions, such as substance and attribute, cause and effect, &c., unites groups of sensations or sensible ideas into objects and events, e.g. a house, one ball moving another; and that, accordingly, perception and experience, requiring both sense and understanding, are partly a posteriori and partly a priori, and constitute a knowledge of objects which, being sensations combined by synthetic unity under a priori forms, are more than mere sensations, but less than things in themselves. This first position is psychological idealism in a new form and supported by new reasons; for, if experience derives its matter from mental sensations and its form from mental synthesis of sensations, it can apprehend nothing but mental objects of sense, which, according to Kant, are sensible ideas having no existence outside our thought, not things in themselves; or phenomena, not noumena.
b. As to the known world, Kant’s position was the logical deduction that from such phenomena of experience all we can know by logical reason is similar phenomena of actual or possible experience; and therefore that the known world, whether bodily or mental, is not a Cartesian world of bodies and souls, nor a Spinozistic world
of one substance, nor a Leibnitzian world of monadic substances
