A serious objection to this hypothesis lies in the discontinuity and irregularity of the mental series. Every hour of deep sleep interrupts it. At one moment we have only a few dull, monotonous sensations, at another a multitude of clear and articulate ideas, accompanied by conflicting emotions. These incalculable fluctuations make it impossible for us to regard the content of one moment as the adequate cause of the content of the next. Even if memory, thought, and will could be explained as results of previous states of consciousness, no explanation could be given of such an absolutely new fact as a fresh sensation or perception.
The only conceivable way of constructing a continuous causal series on the mental side, so as to explain all mental states as effects of previous mental states, would be to assume the existence of mental states of which we are not conscious, and use these to fill out the lacuna of consciousness. This assumption is, as a matter of fact, illegitimate; for it belongs to the very essence of a mental state to be present to consciousness, and an unconscious mental state therefore involves a contradiction in terms. But if for argument's sake we nevertheless make it, and suppose that not only the molecular motions that go on in our brains but all motions of matter are accompanied by some kind of mental state; if, that is to say, we look upon the material world as only an outer shell covering an infinitude of conscious, semi-conscious, and unconscious mental states — it still remains true that these states are all mine; no provision has been made for the existence of another consciousness than my own; and I am confined in the barrenest solipsism.
In short, a consistent spiritualistic theory of the relations of mind and body is simply impossible, and we therefore turn to the only hypothesis which remains — the hypothesis according to which the physical series is the condition of the mental. This means that physical events take place independently, according to laws of their own, and that a few of them — namely, certain events in the cortex — are the indispensable condition for the occurrence of mental events. The psychophysical materialism which this doctrine asserts is to be sharply distinguished from philosophical materialism, or the doctrine that the ultimate reality of the world is matter, a frequent but wholly unjustifiable inference from it.
Psychophysical materialism is often supposed to be correctly expressed in the statement that brain-events cause states of consciousness. This is an illegitimate use of the word cause. For the statement would mean, if true, that what was at one moment a molecular motion in the brain was at the next a state of consciousness, and that the transition from one to the other could be continuously imagined — a manifest absurdity. Causality can only connect physical with physical, not phys-
