The question whether it is possible to obtain precise causal laws in which the causes are psychological, not material, is one of detailed investigation. I have done what I could to make clear the nature of the question, but I do not believe that it is possible as yet to answer it with any confidence. It seems to be by no means an insoluble question, and we may hope that science will be able to produce sufficient grounds for regarding one answer as much more probable than the other. But for the moment I do not see how we can come to a decision.
I think, however, on grounds of the theory of matter explained in Lectures V and VII, that an ultimate scientific account of what goes on in the world, if it were ascertainable, would resemble psychology rather than physics in what we found to be the decisive difference between them. I think, that is to say, that such an account would not be content to speak, even formally, as though matter, which is a logical fiction, were the ultimate reality. I think that, if our scientific knowledge were adequate to the task, which it neither is nor is likely to become, it would exhibit the laws of correlation of the particulars constituting a momentary condition of a material unit, and would state the causal laws* of the world in terms of these particulars, not in terms of matter. Causal laws so stated would, I believe, be applicable to psychology and physics equally; the science in which they were stated would succeed in achieving what metaphysics has vainly attempted, namely a unified account of what really happens, wholly true even if not the whole of truth, and free from all convenient fictions or unwarrantable assumptions of metaphysical entities. A causal law applicable to particulars would count as a law of physics if it could be stated in terms of those fictitious systems of regular appearances which are matter; if this were not the case, it would count as a law of psychology if one of the particulars were a sensation or an image, i.e. were subject to mnemic causation. I believe that the realization of the complexity of a material unit, and its analysis into constituents analogous to sensations, is of the utmost importance to philosophy, and vital for any understanding of the relations between mind and matter, between our perceptions and the world which they perceive. It is in this direction, I am convinced, that we must look for the solution of many ancient perplexities.
* In a perfected science, causal laws will take the form of
differential equations—or of finite-difference equations,
if the theory of quanta should prove correct.
It is probable that the whole science of mental occurrences, especially where its initial definitions are concerned, could be simplified by the development of the fundamental unifying science in which the causal laws of particulars are sought, rather than the causal laws of those systems of particulars that constitute the material units of physics. This fundamental science would cause physics to become derivative, in the sort of way in which theories of the constitution of the atom make chemistry derivative from physics; it would also cause psychology to appear less singular and isolated among sciences. If we are right in this, it is a wrong philosophy of matter which has caused many of the difficulties in the philosophy of mind—difficulties which a right philosophy of matter would cause to disappear.
The conclusions at which we have arrived may be summed up as follows:
I. Physics and psychology are not distinguished by their material. Mind and matter alike are logical constructions; the particulars out of which they are constructed, or from which they are inferred, have various relations, some of which are studied by physics, others by psychology. Broadly speaking, physics group particulars by their active places, psychology by their passive places.
II. The two most essential characteristics of the causal laws which would naturally be called psychological are SUBJECTIVITY and MNEMIC CAUSATION; these are not unconnected, since the causal unit in mnemic causation is the group of particulars having a given passive place at a given time, and it is by this manner of grouping that subjectivity is defined.
III. Habit, memory and thought are all developments of mnemic causation. It is probable, though not certain, that mnemic causation is derivative from ordinary physical causation in nervous (and other) tissue.
IV. Consciousness is a complex and far from universal characteristic of mental phenomena.
V. Mind is a matter of degree, chiefly exemplified in number and complexity of habits.