Logic in Practice

L. Susan Stebbing

• Publisher: Routledge
• Year: 2019
• ISBN:
  9781000737158
• Edition: 1
• Topic: Philosophy
• Subtopic: Philosophy History & Theory

Originally published in 1934. This fourth edition originally published 1954., revised by C. W. K. Mundle.

"It must be the desire of every reasonable person to know how to justify a contention which is of sufficient importance to be seriously questioned. The explicit formulation of the principles of sound reasoning is the concern of Logic".

This book discusses the habit of sound reasoning which is acquired by consciously attending to the logical principles of sound reasoning, in order to apply them to test the soundness of arguments. It isn't an introduction to logic but it encourages the practice of logic, of deciding whether reasons in argument are sound or unsound. Stress is laid upon the importance of considering language, which is a key instrument of our thinking and is imperfect.

CHAPTER I

PURPOSIVE THINKING

‘Where the senses fail us reason must step in.’—GALILEO

THINKING is an activity; we think in order to do. But not all doing consists in overt action producing perceptible changes in the given situation or environment. The ‘man of action’ is commonly opposed to the ‘man of thought’. There are good grounds for this opposition; but even men of action have to think, however much their activities may suggest the contrary. The world today needs clear thinkers even more than it needs men of good will, and not less than it needs men of great practical energy. To be confronted with a problem is to be compelled to think. Thinking essentially consists in asking questions and attempting to answer them. To ask a question is to be conscious of a problem; to answer correctly is to have discovered its solution. Purposive thinking is thinking directed to answering a question held steadily in view. Such directed thinking may be contrasted with idle reverie.¹

Suppose a man lying awake in his cabin on board a passenger steamer. He listens to the sound of the sea, to the numerous slight sounds—the creakings and strainings—always audible on board ship. His hearing of these sounds may partly determine the flow of his thoughts; he passes idly from one thought to another. Suddenly he, hears a loud, distinctive sound—the three long booms which are the danger-signal. This sense-impression is significant; he does not notice it merely as a sound; it signifies for him—ship in danger. He springs up, snatches a coat, and rushing out hears the word ‘Fire!’ The reader’s imagination may supply the details. Provided that the man be not too panic-stricken to think at all, his thinking will now be purposive; it will be directed to securing his own safety or that of others. He will now actively connect one apprehended fact with another. Once the fire-situation is grasped, his thinking will be directed to a practical end; the conditions of attaining this practical end will constitute the problem which his thinking is directed to solving.

¹ Cf. C. A. Mace: The Psychology of Study, Chap. IV. In this chapter will be found an excellent account of the process of thinking, treated from the point of view of the psychologist.

Suppose now that a committee of investigation is confronted with the problem of how the fire originated. This problem is purely theoretical, however much the desire to solve it may be the practical desire of assessing the responsibility for the outbreak of fire, or of attempting to prevent the occurrence of such accidents in the future. A problem is not made a practical problem simply because its solution may have practical applications. The committee are seeking to obtain knowledge; they want to find a true answer to a definite question. Their problem is as purely theoretical as the question of determining the conditions of combustion in general, or the problem of determining the nature of eclipses. The distinction between what is often called practical thinking and theoretical thinking lies wholly in the purpose for which the thinking is pursued. In both cases the thinking process is the same; it is purposive, and thus directed. The opposition is not between practical and theoretical thinking but between directed thinking and idle reverie.

Intelligent dealing with a problem involves, first, the apprehension of a situation giving rise to the problem; secondly, the explicit awareness of the question constituting the initial stage of the problem; thirdly, formulation of the conditions to which the solution must conform. These conditions are determined by the total situation. In so far as these conditions are clearly apprehended and selectively attended to, precise questions can be formulated and various answers tried out. The point to be stressed is that to ask an intelligent question is to have taken note of the conditions set by the problem; to suggest an intelligent answer is to have discerned within the situation, so far as apprehended, those factors which may be relevant to the solution. Intelligent answers may be wrong, but they are never off the point. To keep to the point is to be guided by relevant considerations alone.

The importance of excluding irrelevant suggestions cannot be overstressed. In the case of the man on the burning ship, it is clear that his thinking would be effective only in so far as what came into his mind—or, at least, was taken note of—was relevant to the conditions constituting his problem. If he were to consider whether he could fly like a bird from the ship, or whether the flames could be extinguished by a heavy rainfall, he would be asking questions the answers to which could have no bearing upon the difficulty with which he was confronted. In the same way the committee investigating the cause of the fire would be making no progress towards the solution of their problem if they were to ask whether the fire was due to the ship’s having sailed from port on a Friday, or whether it was caused by a hot dispute between members of the crew, or whether it was due to the wrath of God because the passengers were dancing on a Sunday. That these suggestions at once strike the reader of this book as absurd is due to his knowing too much about the kind of situation, and thus about the conditions of the problem under discussion, even to entertain the possibility that such factors could be relevant. It might well be relevant, however, for the committee to ask whether the fire was due to the careless dropping of a lighted match or of an unextinguished cigarette end, or to a faulty electrical cable, or to a deliberate act of incendiarism. Each of these questions would suggest other questions the answers to which might more easily be ascertained to be correct or incorrect. In this way progress towards a solution may be made. We may briefly consider each of the last three suggestions in order to see how a relevant answer may admit of testing.

The question the committee sought to answer was quite definite, namely, What caused this fire? They sought to discover, not a possible cause of fire on board ship, but the actual cause of this fire. Each suggestion presents a possible cause. Other possible causes might have been considered. The only way to test these suggestions is to ask what else would have happened if the given suggestion were correct, but would probably not have happened if one of the other suggestions were correct. To be able to ask these further questions the committee would need to know a good deal about the ship and about what took place when the fire occurred; they would also have to possess certain technical knowledge. If the ship had been completely burnt out, some relevant questions could not be answered. If, however, the burning ship had been towed into port or there were survivors who could answer definite questions, and if the committee possessed other relevant knowledge, it might be possible to say what was most probably the cause. These conditions may be assumed to be fulfilled, for without some means of obtaining such information there would be nothing to investigate.

Each of the suggestions put forward is a supposition to the effect that some occurrence happened, this occurrence being such that, if it had happened, then fire would have broken out. A supposition thus entertained in order to account for what happens is called an hypothesis. The committee seriously considered three hypotheses. Each hypothesis has certain consequences, i.e. given that the hypothesis were correct something else would have happened. The question then arises whether it did happen.

(1) The hypothesis that the fire was caused by a carelessly dropped lighted match suggests the following questions: (a) Did the fire begin in a cabin or in a public part of the deck? (b) Did it occur at night? If the answer to (b) is affirmative, and if the answer to (a) is that the fire broke out in the luggage-room, then it is improbable that this first hypothesis is correct, since it is unlikely that any one would be smoking in the luggage-room, especially at night.

(2) The hypothesis that the cause was a fault in an electrical cable suggests the questions: (c) Did the fire originate in the vicinity of an electrical cable? (d) Had the electrical installation in this part of the ship been recently overhauled so that it was known to be in good repair? (e) Did anyone notice, at the time when the fire broke out, that in some parts of the ship the electrical supply was cut off, indicating a blown fuse? An affirmative answer to (c) would not of course rule out the lighted match hypothesis, but if the fire had broken out at night and in a little frequented part of the ship, then the fact that it originated in the vicinity of an electric cable would tell in favour of the present hypothesis. But if the answer to (d) were ‘Yes’ and to (e) were ‘No’, then the available evidence would not be in favour of this hypothesis. At this stage the third hypothesis seems plausible, namely, that some one deliberately set the ship on fire.

(3) This hypothesis is not one which would readily be accepted. Careless smokers and electrical faults are familiar enough. Certainly incendiarists are not unknown. But no sane, ordinary passenger would put himself to the inconvenience, perhaps even peril, of being on a burning ship. Thus, unless the incendiarist were a madman, he must have had some strong motive for so acting. This hypothesis suggests the question whether any one on board could have expected to derive some benefit from the destruction of the ship, or was the agent of some one who had such expectations. In attempting to answer this question the committee would be led to pursue investigations beyond the occurrences on the voyage itself. They would ask what benefit could be derived from the destruction of the ship, and who would be benefited. Further questions at once arise. (f) Was the ship heavily insured? (g) What was the age of the ship? (h) Were the owners in need of money? Suppose it were found that the ship was heavily insured; that its future sea-going life was not likely to be long; that the owners needed the insurance money; then this third hypothesis is worth taking seriously. If it were further discovered that other ships belonging to the same Line had been recently destroyed by fire, then the hypothesis will seem not unreasonable under the circumstances.

It is not worth while to pursue this illustration further. Enough has been said to show how an intelligent person confronted with a problem will proceed to ask questions and guess at the answer; how various answers lead to other questions and further guesses. A guess is worth making only if the answer can be tested. These guesses are suggestions as to what may possibly be the case; i.e. they are hypotheses. These hypotheses are worth entertaining only if the possibilities can be narrowed down. The possibilities can be narrowed down by discovering that what would be possible in certain other circumstances is not possible under the ascertained circumstances. In the next chapter we shall see how such an investigation conforms to certain principles which interest the practical logician.

Thinking out a problem involves inference. Inference is a mental process in which a thinker passes from something given or taken for granted—the datum—to something which he accepts because; and only because, he has accepted the datum. It is a passage of thought from datum to conclusion. To accept a conclusion as the result of an inference is to accept it upon the basis of what is taken to be evidence. To regard what is observed, or is believed, or is in any way apprehended, as evidence is to regard it as indicating something other than itself. To regard a fact as providing evidence is, then, to regard that fact as significant of some other fact. We may have some evidence in favour of a conclusion and no evidence contrary to it, and yet the evidence may not be decisive. Evidence is decisive for a certain conclusion when the acceptance of the evidence necessitates the acceptance of the conclusion. Unfortunately we may be mistaken both with regard to what the facts are and with regard to what the facts indicate.

The example of the committee’s investigation has shown us that intelligent guessing is controlled by the recognition of certain ascertained conditions as relevant to the solution of the problem. These conditions relate to matters of fact. They may be called the material conditions of the problem since they are provided by the special subject-matter of the investigation. To apprehend a condition as relevant to the solution is already to know a certain amount about the nature of situations resembling the given one in certain respects. If we knew all about the situation there would be no problem. If we knew nothing about similar situations we should not be able even to ask intelligent questions about it. Relevance is significance for the question at issue. Nothing is significant in itself. That a ship is heavily insured acquires significance if the ship belongs to owners in financial difficulties and if its destruction occurred in a manner compatible with a deliberate act of incendiarism; it ceases to be significant if the owners are known to be men of incorruptible honesty. A red light at a street corner signifies ‘stop’ to a motorist who knows the conventions of traffic regulation. A certain-shaped mark on the sand is significant of the previous presence of a human being only to one who can interpret the mark as a footprint. A grey mark on a carpet may be significant of a certain brand of cigars to one who, like Sherlock Holmes, has made a special study of varieties of cigar ash. To multiply examples is unnecessary. The point must be obvious to any one on reflection. Nevertheless, we are apt to forget that significance arises only in so far as a given fact indicates something other than itself. The possibility of such indication depends upon knowledge possessed by the person apprehending the significance. The detective who ‘takes in the whole situation at a glance’ would need already to know what each item he observes signifies. In sober life this is not the case. The observed facts acquire significance only when viewed in the light of a definite question which guides his thinking. Reviewed in the light of a different question, the same facts may alter in significance. Readers familiar with detective novels will be able to supply examples illustrating this point.

It should now be clear that significance is a property of signs. A sign indicates something other than itself. Signifying is a relation requiring three terms: a sign, that of which the sign is significant, and a thinker for whom the former indicates the latter. Just as a book, for example, is not a gift unless it be given by some one to some one, so a red light, for example, is not a sign unless it be interpreted by some one to indicate something. To be able thus to interpret a presented fact we must know something not given in the presented situation.

It is a mistake to regard thinking which involves interpretation of signs as confined to the higher levels of consciousness, or to confine ‘problems’ to what are often called intellectual problems. There are no. hard and fast distinctions in the development of mental life. The distinction between practical and theoretical problems does not relate to the kind of thinking involved, but to the set of purposes within which the problem originates, and hence to the kind of changes its solution is designed to bring about. A problem may be practical in the sense that it relates to bringing about a change in the environment, for example, making a box, or mending a clock, or disposing an army, or winning an election campaign. A problem may be theoretical in that it relates to bringing about a change in the mind of a thinker who has no other purpose in view than to apprehend a situation more clearly by discerning the connexion between its component elements. The purpose of solving the problem is, in the latter case, the acquirement of knowledge; in the former, the doing of something. In both alike, thinking is directed to an end determined by the nature of the problem. In effective thinking consciousness of the conditions set by the problem-situation directs the cognitive activity of the thinker, determining what shall come, to mind.

Directed thinking in its most highly developed form is reasoning. To be reasonable is to be capable of apprehending a situation as a whole, to take note of those items which are relevantly connected, and not to connect arbitrarily items not apprehended as significant. Apprehension of relevance depends upon two quite different factors: knowledge and sagacity. ‘To be sagacious’, William James has said, is to be a good observer.’ This statement may be accepted if we admit that a good observer is one capable of discerning relevant connexions. Certainly a good observer is not one who ‘stands and stares’. Staring is a sign of stupidity, or of an astonishment so profound as temporarily to destroy the power to think reflectively. in reasoning we select from a set of items presented to us just those which are significant in relation to the facts we are trying to establish. Failure to be reasonable may occur in one or other of two very different ways. Significance may be missed through sheer ignorance or through passion and emotional attitudes which check clear-sighted apprehension. No doubt we all desire to be reasonable but few of us achieve this desire. Even those few are reasonable only at rare moments. Ignorance and passion present grave obstacles to be ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Original Title Page
• Original Copyright Page
• Dedication
• Epigraph
• Preface
• Contents
• Chapter I. Purposive Thinking
• Chapter II. The Importance of Form
• Chapter III. Deductive Forms
• Chapter IV. Ambiguity, Indefiniteness, and Relevance
• Chapter V. The Estimation of Evidence
• Chapter VI. The Grounds of our Beliefs
• References for Reading
• Index