John Venn, pp. 112-117 of Principles of Inductive Logic
Essential philosophy, IMO.
The Principles of Empirical or Inductive Logic, by John Venn, pp. 112-117
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p. 112
CHAPTER V.
THE SUBJECTIVE FOUNDATIONS OF INDUCTION.
We have now examined, with sufficient minuteness and care for our present purposes, what may be called the objective or material foundations of an Inductive system of Logic. But, in accordance with the general view already insisted on, such an examination deals only with one side of our subject. Logic is neither a purely objective nor a purely subjective science, but essentially and almost exclusively a science which involves both aspects. It concerns itself with the operations of the human mind when drawing inferences about the phenomena of nature. Accordingly we must now enter into some examination of the second, or mental, side of the enquiry, by ascertaining the nature of the postulates which have to be demanded from the regions of Psychology or Metaphysics before a System of Inference can be constructed.
I. In the first place; it is obvious that the ordinary powers of observation must be taken for granted. Logic, by universal admission, in every application we make of it, starts from premises which have been obtained from observation, directly or remotely. We must therefore include, amongst our postulates, the existence of these powers of observation. As however this is in no way peculiar to Logic, but applies in an equal or even greater degree to many of the special sciences, we need not pause to examine it as a general postulate.
Where the question does force itself upon our notice, — and indeed, as we are about to see, raises some very perplexing problems, — is not so much in respect of the mere assumption of these powers, or in the assignment of their general character, but rather in the attempted determination of their boundary line. Where, in fact, are we to suppose that pure Observation
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ends and true Inference begins? In a Science of Inference such a question as this is a serious one ; and it must be frankly admitted that any doubts and difficulties which we encounter in answering it constitute a flaw in the theoretic perfection of the science. Unfortunately however there seems no way of completely removing such doubts, and all that we can do is to minimize their consequences.
Any simple example will serve to illustrate the difficulty. Suppose I am on a walking tour, and a stranger proposes to join our party; I give a glance at him and say to my friend, 'I can see plainly enough that he will not be fit for our excursion to-day’. Now though this remark is couched in the language of mere observation any one uttering it would not need to be reminded that it is a mixture of observation and inference ; and if he spoke with less colloquial abbreviation he would intimate the distinction by expressing himself somewhat as follows, — ‘ I can see that the man is ill, and therefore I am sure he cannot take a long walk ‘. In common parlance the present illness is an observation, and the inability to take the walk is an inference. We might not be consciously thinking of the distinction at the time, but this is the sort of analysis we should instantaneously make when attention was directed to the point. Our plain man would reply, ' you can see for yourself the state he is in. Just look at him, how ill he is ‘, and so forth.
Now it is a merely elementary step in analysis to point out that the assumed state of the man, bodily and mental, which is involved in the ‘ illness ‘, is largely a conclusion founded on data. The very expression ‘ symptom ‘, so commonly applied to diseases, is an illustration that the distinction has been recognized as far as this by all but the rudest and most unobservant. So far then we have pushed the observation a stage further back, having resolved it into such elements as the paleness, the lax or stooping gait, perhaps the quickness of breathing, and so forth, which are considered to be the symptoms of the disease.
But then begins again the never-ending process of analysis as applied to these elements themselves. For shortness, take but one of these, the paleness, where we are purposely confining ourselves to a characteristic which seems about as simple and elementary as experience can furnish : it is one of colour pure and simple. But the psychologist has something to say about
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this. It admits of simple proof that the colour of the man's face, as perceived by us, varies vastly more according as we see it by daylight or candlelight, or even according as he stands somewhat more or less in the shade, than it can possibly vary according to the extremest conditions of health and sickness whilst the light remains the same. That is, our subjective estimate of such a simple and apparently ultimate datum as this of mere colour is in great part an instinctive judgment or inference. What we really saw is so instantly corrected and allowed for that it actually drops out of notice, whilst what is effectively retained is something so different from the former that it must be regarded as very largely consisting of inference.
Again; suppose that by an effort of reflection, and comparison of the same shade under varying conditions, we had enabled ourselves to estimate the colour as it was, that is, as it should be under normal circumstances, — and the psychologist knows how difficult this would be, — was it really true that we saw, as we supposed, a surface of that colour? It is highly unlikely that we did so. What any ordinary glance takes in, when directed towards a surface, is nothing more than a succession of points which are supplemented and filled in by something else than sight. At least this is all that is perceived by the central spot of the retina, which alone is capable of clear vision. How obstinately our senses refuse to undertake the drudgery of examining every separate detail in the objects we inspect, even when we are gazing upon them with some care, is only too well known to those who have ever worked through a proof sheet as it came from the press. The almost inevitable impulse is to visualize a few letters and thence to infer the whole word, and even from a part of a sentence to infer the rest ; and it requires a strong and persistent effort to insist that the eye shall not thus shirk its work of adequate observation.
Finally; take as minute a fragment of visible area as we choose, so as to avoid any such spatial filling in as that just indicated : is the impression really continuous, either in time or space? Confine ourselves, for the sake of brevity, to the former continuity. It is approximately certain in the case of sight, and quite certain in the case of sound, that what seems to us to be
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a continuous elementary impression is really made up of distinct nervous impulses or shocks. We are not referring here to the fact, familiarly illustrated by the case of a rapidly revolving point of light, that finite impressions outlast their producing cause, and so tend, when repeated after short intervals, to overlap and become continuous. We are here going a stage further back, and are enquiring into the mode of production of the most elementary and briefest of such finite impressions themselves; that is, we are referring to the process by which impulses or shocks which separately do not emerge into consciousness can yet do so when there is a sufficient succession of them. The fact itself must of course be taken for granted here; the only question now before us being whether the distinction between datum and inference which has been pushed thus far back, is to be considered capable of receding one stage further still. There are many psychologists who distinctly claim these non-conscious elements as being as truly ' mental ‘ as those of which we are conscious ; are we then to admit that the step from the one to the other is to be regarded as a logical step, and as being of the nature of inference ?
As we have not yet come to examine into the real nature of inference in the cases in which its existence is undisputed, it would be impossible to attempt to decide this question properly here. We will merely indicate in a few words why such a step as this last is not to be ranked as a logical one. Briefly, then, Logic is concerned, not necessarily with processes of which we are conscious at the time, — for many unquestionable inferences take place spontaneously, and without our being aware at the time that they are such, — but at any rate with those that can be voluntarily reproduced when attention is directed to them. This seems the most definite and convenient point at which to mark the line. In all the successive cases indicated in the foregoing description, except the last, the process seems essentially to be of the same character. We had mentally taken a definite step from one conscious element to another: often no doubt without knowing that we had done so : but it was always a step which we could, if we pleased, go over again deliberately. We felt that we could revise or justify our judgment. But the step which leads us into consciousness is a very different one from that which only leads us from one point
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to another within its province. The data here were such that no amount whatever of introspection could possibly set them before us directly: we can only reach them indirectly by analogy, not start from them deliberately.
The general conclusion I should draw in respect of our attitude towards any really ultimate data, is that they can no more be reached than can a first point or absolute limit in time or space. Everywhere, however far back we may succeed in pushing our analysis, we find ourselves in the same general position: — that of having something in hand which implies something beyond or behind it. The metaphor of the ever-receding horizon which we follow in vain if we seek to find a terminus there, seems a sound one. We cannot start from the horizon and work our way steadily from this as a beginning, up to the point at which we now stand : our path is in the opposite direction, ever straining towards something which it is impossible for us actually to attain.
The popular estimate of the claims of Logic is, presumably, that it has a definite starting point : that if we do not attain ultimate data it is merely because we have not taken the trouble to go back to them : that sense or intuition can always furnish them for us. This view is supported by a stock of common metaphors, which, whether they conceive our path to be an upward or a downward one, whether, that is, they liken it to a chain hanging down, or to a building rising up, always suggest a definite starting point. The links must have some fixed attachment, whence they hang firmly : the courses of masonry must have a solid foundation, on which they rest securely ; and so forth. All these metaphors are misleading, unless it be expressly explained that any such starting point is a merely conventional one, assumed for convenience. Everywhere, wherever we may happen to find ourselves, we are in possession of data which are familiar to us and are justified by experience. These are our starting point, and not any really primitive data. From thence we proceed, so to say, outwards, always striving towards absolute origins or elementary data, but without the slightest hope of ever reaching them.
The attitude of most ordinary persons towards the distinction between observation and inference is quite in harmony with this view. They do not indeed deliberately recognize that no
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ultimate elements can ever be obtained; they do not much trouble themselves about any such consideration. What they primarily have in view is not the distinction between observation and inference, but rather that between what may conveniently be taken for granted and what needs reasons for its support. The two distinctions are not quite the same thing, but they run nearly parallel. When, in our example some pages back, the speaker says that he can see that the stranger is not fit for the expedition, all that he has in view is that such an opinion will be readily accepted. On this being questioned he falls back on the statement that he can see the man is ill, claiming that this at least will pass without question. And so on, step by step. He is not thinking of anything so technical as ‘pure observation' and where exactly this may be detected, he is only thinking of what will be admitted then and there by those to whom he is speaking; and he is prepared to go as far back, step by step, as may reasonably be expected until he and they come to some common basis of agreement. But he would naturally soon become irritated with any one who kept up the analytical cross-examination too long, on the ground that it was quibbling about points which no rational person could doubt.
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