Our discussions, hitherto, have been concerned very largely with Man, but Man on his own account is not the true subject-matter of philosophy. What concerns philosophy is the universe as a whole; Man demands consideration solely as the instrument by means of which we acquire knowledge of the universe. And that is why it is human beings as capable of knowledge that have concerned us mainly in past chapters, rather than as centres of will or of emotion. We are not in the mood proper to philosophy so long as we are interested in the world only as it affects human beings; the philosophic spirit demands an interest in the world for its own sake. But since we apprehend the world through our own senses, and think about it with our own intellect, the picture that we acquire is inevitably coloured by the personal medium through which it comes to us. Consequently we have to study this medium, namely ourselves, in order to find out, if we can, what elements in our picture of the world are contributed by us, and what elements we may accept as representative of outside fact. Previous chapters have studied cognition, both as an outwardly observable reaction, and as it appears to introspection. In the chapters that remain, we shall be concerned with what we can know about the universe, in view of the nature of the instrument that we have to employ. I do not think we can know as much as many philosophers of the past have supposed, but I think it is worth while to have in our minds an outline of their systems. I shall therefore begin by setting forth a few typical philosophical constructions of earlier centuries.
Modern philosophy is generally taken as beginning with Descartes, who flourished in the first half of the seventeenth century. We have already had occasion, in [Chapter XVI], to consider his argument “I think, therefore, I am”, but now we will deal with him somewhat more generally. He inaugurated two movements, one in metaphysics, one in theory of knowledge. In metaphysics, he emphasised the gulf between mind and matter, or between soul and body; in theory of knowledge he advocated a critical scrutiny of premises. These two movements had different histories, each of them interesting. The science of dynamics was rapidly developing in Descartes’ time, and seemed to show that the motions of matter could be calculated mathematically, given sufficient data. As the motions of matter include our bodily acts, even speaking and writing, it seemed as if the consequence must be a materialistic theory of human behaviour. This consequence, however, was distasteful to most philosophers, and they therefore invented various ways of escaping from it. Descartes himself thought that the will could have certain direct physical effects. He thought that the brain contains a fluid called the “animal spirits”, and that the will could influence the direction of its motion, though not the velocity. In this way he was still able to hold that the will is effective in the manner in which common sense supposes it to be. But this view did not fit in at all well with the rest of his philosophy. He held that, apart from the Supreme Substance, namely God, there are two created substances, mind and matter; that the essence of mind is thought, and that the essence of matter is extension. He made these two substances so different that interaction between them became difficult to understand, and his followers decided that there is never any effect either of mind on matter or of matter on mind.
The motives for this development were various; perhaps the most important was the development of physics immediately after Descartes’ time. A law was discovered called the “conservation of momentum”. This states that, if a system of bodies is in any sort of motion, and is free from outside influences, the amount of motion in any direction is constant. This showed that the kind of action of the will on the “animal spirits” which Descartes had assumed was contrary to the principles of dynamics. It seemed to follow that mind cannot influence matter, and it was inferred that matter cannot influence mind, since the two were regarded as co-equal substances. It was held that each goes it own way, according to its own laws. The fact that our arm moves when we will it to move was regarded as analogous to the fact that two perfectly accurate clocks strike at the same moment, though neither has any effect upon the other. The series of mental events and the series of physical events were parallel, each going at the same rate as the other, therefore they continued to synchronise, in spite of their independence of each other.
Spinoza sought to make this parallelism less mysterious by denying that there are two separate substances, mind and matter. He maintained that there is only one substance, of which thought and extension are attributes. But there seemed still no good reason why the events belonging to the two attributes should develop along parallel lines. Spinoza is in many ways one of the greatest philosophers, but his greatness is rather ethical than metaphysical. Accordingly he was regarded by contemporaries as a profound metaphysician but a very wicked man.
The notion of the impossibility of interaction between mind and body has persisted down to our own day. One still hears of “psychophysical parallelism”, according to which to every state of the brain a state of mind corresponds and vice versa, without either acting on the other. This whole point of view, though not exactly that of Descartes, derives from him. It has a number of sources, religious, metaphysical, and scientific; but there seems no ground whatever for regarding it as true.
Take, first, the rigid determinism of traditional physics, which was to have been avoided. Spinoza rightly perceived that this could not be avoided by such methods, and therefore accepted determinism in the psychical as in the physical realm. If everything we say is determined by physical causes, our thoughts are only free when we tell lies: so long as we say what we think, our thoughts also can be inferred from physics. The philosophy which I advocate escapes this consequence in several ways. In the first place, causality does not involve compulsion, but only a law of sequence: if physical and mental events run parallel, either may with equal justice be regarded as causing the other, and there is no sense in speaking of them as causally independent. Thus the Cartesian dualism does not have the pleasant consequences which were intended. In the second place, modern physics has become less deterministic than the physics of the past few centuries. We do not know, e.g. what makes a radio-active atom explode or an electron jump from a larger to a smaller orbit. In these matters we only know statistical averages.
Take next the view that mind and matter are quite disparate. This we have criticised already. It rests upon a notion that we know much more about matter than we do, and in particular upon the belief that the space of physics can be identified with the space of sensible experience. This belief is absent in Leibniz, who, however, never quite realised what his own view was. It is not absent in Kant, who realised that the space of sensible experience is subjective, and inferred that the space of physics is subjective. Since Kant, no one seems to have thought clearly about space until Einstein and Minkowski. The separation of physical and sensible space, logically carried out, shows the groundlessness of traditional views about mind and matter. This part of Descartes’ philosophy, therefore, though it accelerated the progress of physics, must be regarded as metaphysically an aberration.
The other part of Descartes’ philosophy, namely, the emphasis upon methodical doubt, and consequently upon theory of knowledge, has been more fruitful. The beginning of a philosophic attitude is the realisation that we do not know as much as we think we do, and to this Descartes contributed notably. We have seen that he set to work to doubt all he could, but found he could not doubt his own existence, which he therefore took as the starting-point of his constructive system. He supposed that the most certain fact in the world is “I think”. This was unfortunate, since it gave a subjective bias to modern philosophy. As a matter of fact, “I” seems to be only a string of events, each of which separately is more certain than the whole. And “think” is a word which Descartes accepted as indefinable, but which really covers complicated relations between events. When is an event a “thought”? Is there some intrinsic characteristic which makes it a thought? Descartes would say yes, and so would most philosophers. I should say no. Take, e.g. a visual and an auditory sensation. Both are “thoughts” in Descartes’ sense, but what have they in common? Two visual sensations have an indefinable common quality, viz. that which makes them visual. Two auditory sensations likewise. But a visual and an auditory sensation have in common, if I am not mistaken, no intrinsic property, but a certain capacity for being known without inference. This amounts to saying that they are mnemic causes of a certain kind of event, called a cognition, and that they have moreover, a certain formal similarity to the cognition which they cause. Therefore, instead of taking the general “I think” as our basis, we ought to take the particular occurrences which are known without inference, among which sensations (or rather “perceptions”) will be included. These occurrences, as we have already seen, may be regarded with equal justice as physical and mental: they are parts of chains of physical causation, and they have mnemic effects which are cognitions. The former fact makes us call them physical, the latter mental, both quite truly. It is the particular events which are certain, not the “I think” which Descartes made the basis of his philosophy. It is not correct to regard the ultimate certainties as “subjective”, except in the sense that they are events in that part of space-time in which our body is—and our mind also, I should say.
A new turn was given to the Cartesian type of metaphysics by Leibniz (1646–1716), who, like Descartes, was supremely eminent both in mathematics and in philosophy. Leibniz rejected the view that there is only one substance, as Spinoza held, or only two other than God, as the orthodox followers of Descartes maintained. He also rejected the dualism of mind and matter, holding that there are innumerable substances all in a greater or less degree mental, and none in any degree material. He maintained that every substance is immortal, and that there is no interaction between one substance and another—this last being a view derived from the Cartesian independence of mind and matter. He also extended to his many substances the belief in parallelism which had existed for the two substances of the Cartesians. He called his substances “monads”, and maintained that every monad mirrors the universe, and develops along lines which correspond, point by point, with those along which every other monad is developing. A man’s soul or mind is a single monad, while his body is a collection of monads, each mental in some degree, but less so than the monad which is his soul. Inferior monads mirror the world in a more confused way than higher ones do, but there is some element of confusion in the perceptions of even the most superior monads. Every monad mirrors the world from its own point of view, and the difference between points of view is compared to a difference of perspective. “Matter” is a confused way of perceiving a number of monads; if we perceived clearly, we should see that there is no such thing as matter.
Leibniz’s system had great merits and great demerits. The theory that “matter” is a confused way of perceiving something non-material was an advance upon anything to be found in his predecessors. He had, though only semi-consciously, the distinction between physical and perceptual space: there is space in each monad’s picture of the world, and there is also the assemblage or pattern of “points of view”. The latter corresponds to what I have called “physical space”, the former to “perceptual space”. Leibniz maintained, as against Newton, that space and time consists only of relations—a view which has achieved a definitive triumph in Einstein’s theory of relativity. The weak point of his system was what he called the “pre-established harmony”, in virtue of which all the monads (so to speak) kept step, in spite of the fact that they were “windowless” and never acted upon each other. Perception, for Leibniz, was not an effect of the object perceived, but a modification arising in the perceiving monad and running parallel with what was happening in the perceived object. This view would never have seemed plausible but for the anterior Cartesian theory of the mutual independence of mind and matter. And if Leibniz himself developed, as he believed, in complete independence of all other created things, it is not clear what good reasons he could have had for believing in the existence of anything except himself, since, by his own theory, his experiences would remain unchanged if everything else were annihilated. In fact, he was only able to refute this possibility by bringing in theological considerations, which, whether valid or not, are out of place in philosophy. For this reason, his doctrines, ingenious as they were, found little acceptance in France and England, though in Germany they prevailed, in a modified form, until the time of Kant.