The concept of causality cannot be worked in psychology in the same way as it can be done in the physical science. The circle of physical processes is complete in itself. A physical process without ceasing to be physical can be traced endlessly in the past or future, all the links of the endless process must all be physical in their nature. For if we permit in the endless chain of links of the physical process any other but physical links to be interpolated, all the physical sciences must fall to the ground, since at any stage we may get hold of a process of which the antecedent link is not of a physical nature. In short, the postulate that forms the basis of physical science is that the antecedent and consequent of a physical process taken at any stage of the process are physical in their nature. This is the principle of continuity. The whole edifice of the physical sciences is based on this principle.
If we now turn to psychology, we find that it cannot be based on a postulate of similar character. Psychology cannot possibly work on the assumption that the processes it deals with can be traced endlessly in either direction, past, or future. Unlike the physical, the psychic process is finite and final,―it has a beginning and an end, it begins with a purpose, conscious, subconscious, or unconscious, and ends with an adjustment. The psychic process begins as a sensation, and its complete cycle runs its course as an idea and then ends in a volition to act. The stimulus marking the beginning of the psychic process and the act marking the end of the process are physical links of a continuous physical process, the links of which can be traced endlessly in physical terms. Taking the psychic process from the ontogenetic standpoint, we find again the same thing. If the psychic life of the individual is taken as a whole and traced backward, in the past, we arrive at some point, when the i stream of consciousness begins, and on following it forward, we finally arrive at a point where the stream of consciousness ends. If we view the question phylogenetically, we come once more to the same conclusion. In the history of biological evolution there was a time when psychic life began, and there will come a time when all psychic life will disappear from our globe. The principle of continuity, the warp and woof of physical science, cannot be worked in psychology, instead of it we can only discover a principle of finiteness and finality.
In a physical process any link taken at random must have a physical antecedent and consequent; not so is it in a psychic process, not each link of the series has its psychic antecedent and consequent, the first link has no antecedent and the last one has no consequent. The phenomena of sleep, of hypnosis, of amnesia, of unconsciousness, of syncope show that the psychic process may be cut short anywhere in its course, and may resume its flow from any given link or stage. The links that go to form the psychic process hang loosely, and any link may really be without an antecedent or without a consequent.
In many cases the seemingly lost antecedent can still be found in the subconscious, dissociated from the active stream of consciousness constituting for the time being the conscious personality or the self-consciousness of the subject, such for instance is the case in the many forms of functional psychopathic diseases and also in hypnosis. In other cases, such for instance, as unconsciousness of epilepsy, the stream of consciousness is interrupted and resumed only after a certain period of time, not even the subconscious can supply us the missing link. In normal sleep we meet once more with an interruption of the current of consciousness, and it is only under certain conditions, such as dreaming, that the subconscious can supply the missing states. Each psychic process is like the life process of a given individual, it has a definite beginning and a definite end; while a physical process has neither beginning nor end, and can be followed out endlessly in the direction of the line of antecedents or consequents. In other words while a physical process is infinite, a psychic process is finite.
Let P be a physical process and p represent a link in that process, then p, p, p, etc., may be represented as its consequents, while p, p, p, p, etc., may be represented as its antecedents. P therefore may be represented by the following infinite series:
The series is infinite in both directions, in direction of p, antecedents, and in the direction of p consequents.
Let S represent a psychic process, s a link in that process, s s s s etc., its consequents s, s, s, s, etc., its antecedents, then the psychic process can be represented by the following series: