Both in introspection and in external perception, we try to express what we know in WORDS.
We come here, as in the question of testimony, upon the social aspect of knowledge. The purpose of words is to give the same kind of publicity to thought as is claimed for physical objects. A number of people can hear a spoken word or see a written word, because each is a physical occurrence. If I say to you, “mind is different from matter”, there may be only a very slight resemblance between the thought that I am trying to express and the thought which is aroused in you, but these two thoughts have just this in common, that they can be expressed by the same words. Similarly, there may be great differences between what you and I see when, as we say, we look at the same chair; nevertheless we can both express our perceptions by the same words.
A thought and a perception are thus not so very different in their own nature. If physics is true, they are different in their correlations: when I see a chair, others have more or less similar perceptions, and it is thought that these are all connected with light-waves coming from the chair, whereas, when I think a thought, others may not be thinking anything similar. But this applies also to feeling a toothache, which would not usually be regarded as a case of introspection. On the whole, therefore, there seems no reason to regard introspection as a different kind of knowledge from external perception. But this whole question will concern us again at a later stage.
As for the trustworthiness of introspection, there is again a complete parallelism with the case of external perception. The actual datum, in each case, is unimpeachable, but the extensions which we make instinctively are questionable. Instead of saying, “I am believing that mind is different from matter”, you ought to say, “certain images are occurring in a certain relation to each other, accompanied by a certain feeling”. No words exist for describing the actual occurrence in all its particularity; all words, even proper names, are general, with the possible exception of “this”, which is ambiguous. When you translate the occurrence into words, you are making generalisations and inferences, just as you are when you say “there is a chair”. There is really no vital difference between the two cases. In each case, what is really a datum is unutterable, and what can be put into words involves inferences which may be mistaken.
When I say that “inferences” are involved, I am saying something not quite accurate unless carefully interpreted. In “seeing a chair”, for instance, we do not first apprehend a coloured pattern, and then proceed to infer a chair: belief in the chair arises spontaneously when we see the coloured pattern. But this belief has causes not only in the present physical stimulus, but also partly in past experience, partly in reflexes. In animals, reflexes play a very large part; in human beings, experience is more important. The infant learns slowly to correlate touch and sight, and to expect others to see what he sees. The habits which are thus formed are essential to our adult notion of an object such as a chair. The perception of a chair by means of sight has a physical stimulus which affects only sight directly, but stimulates ideas of solidity and so on through early experience. The inference might be called “physiological”. An inference of this sort is evidence of past correlations, for instance between touch and sight, but may be mistaken in the present instance; you may, for instance, mistake a reflection in a large mirror for another room. Similarly in dreams we make mistaken physiological inferences. We cannot therefore feel certainty in regard to things which are in this sense inferred, because, when we try to accept as many of them as possible, we are nevertheless compelled to reject some for the sake of self-consistency.
We arrived a moment ago at what we called “physiological inference” as an essential ingredient in the common-sense notion of a physical object. Physiological inference, in its simplest form, means this: given a stimulus S, to which, by a reflex, we react by a bodily movement R, and a stimulus S′ with a reaction R′, if the two stimuli are frequently experienced together, S will in time produce R′.[2] That is to say, the body will act as if S′ were present. Physiological inference is important in theory of knowledge, and I shall have much to say about it at a later stage. For the present, I have mentioned it partly to prevent it from being confused with logical inference, and partly in order to introduce the problem of induction, about which we must say a few preliminary words at this stage.
[2] E.g. if you hear a sharp noise and see a bright light simultaneously often, in time the noise without the light will cause your pupils to contract.
Induction raises perhaps the most difficult problem in the whole theory of knowledge. Every scientific law is established by its means, and yet it is difficult to see why we should believe it to be a valid logical process. Induction, in its bare essence, consists of the argument that, because A and B have been often found together and never found apart, therefore, when A is found again, B will probably also be found. This exists first as a “physiological inference”, and as such is practised by animals. When we first begin to reflect, we find ourselves making inductions in the physiological sense, for instance, expecting the food we see to have a certain kind of taste. Often we only become aware of this expectation through having it disappointed, for instance if we take salt thinking it is sugar. When mankind took to science, they tried to formulate logical principles justifying this kind of inference. I shall discuss these attempts in later chapters; for the present, I will only say that they seem to me very unsuccessful. I am convinced that induction must have validity of some kind in some degree, but the problem of showing how or why it can be valid remains unsolved. Until it is solved, the rational man will doubt whether his food will nourish him, and whether the sun will rise tomorrow. I am not a rational man in this sense, but for the moment I shall pretend to be. And even if we cannot be completely rational, we should probably all be the better for becoming somewhat more rational than we are. At the lowest estimate, it will be an interesting adventure to see whither reason will lead us.
The problems we have been raising are none of them new, but they suffice to show that our everyday views of the world and of our relations to it are unsatisfactory. We have been asking whether we know this or that, but we have not yet asked what “knowing” is. Perhaps we shall find that we have had wrong ideas as to knowing, and that our difficulties grow less when we have more correct ideas on this point. I think we shall do well to begin our philosophical journey by an attempt to understand knowing considered as part of the relation of man to his environment, forgetting, for the moment, the fundamental doubts with which we have been concerned. Perhaps modern science may enable us to see philosophical problems in a new light. In that hope, let us examine the relation of man to his environment with a view to arriving at a scientific view as to what constitutes knowledge.