The question whether there is such a thing as a falsifiable singular statement (or a ‘basic statement’) will be examined later. Here I shall assume a positive answer to this question; and I shall examine how far my criterion of demarcation is applicable to theoretical systems - if it is applicable at all. A critical discussion of a position usually called ‘conventionalism’ will raise first some problems of method, to be met by taking certain methodological decisions. Next I shall try to characterize the logical properties of those systems of theories which are falsifiable - falsifiable, that is, if our methodological proposals are adopted.
conventionalist treats this simplicity as our own creation. For him, however, it is not the effect of the laws of our intellect imposing themselves upon nature, thus making nature simple; for he does not believe that nature is simple. Only the ‘laws of nature’ are simple; and these, the conventionalist holds, are our own free creations; our inventions; our arbitrary decisions and conventions. For the conventionalist, theoretical natural science is not a picture of nature but merely a logical construction. It is not the properties of the world which determine this construction; on the contrary it is this construction which determines the properties of an artificial world: a world of concepts implicitly defined by the natural laws which we have chosen. It is only this world of which science speaks.
According to this conventionalist point of view, laws of nature are not falsifiable by observation; for they are needed to determine what an observation and, more especially, what a scientific measurement is. It is these laws, laid down by us, which form the indispensable basis for the regulation of our clocks and the correction of our so-called ‘rigid’ measuring rods. A clock is called ‘accurate’ and a measuring rod ‘rigid’ only if the movements measured with the help of these instruments satisfy the axioms of mechanics which we have decided to adopt.2
The philosophy of conventionalism deserves great credit for the way it has helped to clarify the relations between theory and experiment. It recognized the importance, so little noticed by inductivists, of the part played by our actions and operations, planned in accordance with conventions and deductive reasoning, in conducting and interpreting our scientific experiments. I regard conventionalism as a system which is self-contained and defensible. Attempts to detect inconsistencies in it are not likely to succeed. Yet in spite of all this I find it quite unacceptable. Underlying it is an idea of science, of its aims and purposes, which is entirely different from mine. Whilst I do not demand any final certainty from science (and consequently do not get it), the conventionalist seeks in science ‘a system of knowledge based upon ultimate grounds’, to use a phrase of Dingler’s. This goal is attainable; for it is possible to interpret any given scientific system as a system of implicit definitions. And periods when science develops slowly will give little occasion for conflict - unless purely academic - to arise between scientists inclined towards conventionalism and others who may favour a view like the one I advocate. It will be quite otherwise in a time of crisis. Whenever the ‘classical’ system of the day is threatened by the results of new experiments which might be interpreted as falsifications according to my point of view, the system will appear unshaken to the conventionalist. He will explain away the inconsistencies which may have arisen; perhaps by blaming our inadequate mastery of the system. Or he will eliminate them by suggesting ad hoc the adoption of certain auxiliary hypotheses, or perhaps of certain corrections to our measuring instruments.
In such times of crisis this conflict over the aims of science will become acute. We, and those who share our attitude, will hope to make new discoveries; and we shall hope to be helped in this by a newly erected scientific system. Thus we shall take the greatest interest in the falsifying experiment. We shall hail it as a success, for it has opened up new vistas into a world of new experiences. And we shall hail it even if these new experiences should furnish us with new arguments against our own most recent theories. But the newly rising structure, the boldness of which we admire, is seen by the conventionalist as a monument to the ‘total collapse of science’, as Dingier puts it. In the eyes of the conventionalist one principle only can help us to select a system as the chosen one from among all other possible systems: it is the principle of selecting the simplest system - the simplest system of implicit definitions; which of course means in practice the ‘classical’ system of the day.3
Thus my conflict with the conventionalists is not one that can be ultimately settled merely by a detached theoretical discussion. And yet it is possible I think to extract from the conventionalist mode of thought certain interesting arguments against my criterion of demarcation; for instance the following. I admit, a conventionalist might say, that the theoretical systems of the natural sciences are not verifiable, but I assert that they are not falsifiable either. For there is always the possibility of ‘.. .attaining, for any chosen axiomatic system, what is called its “correspondence with reality” ’;4 and this can be done in a number of ways (some of which have been suggested above). Thus we may introduce ad hoc hypotheses. Or we may modify the so-called ‘ostensive definitions’
(or the ‘explicit definitions’ which may replace them). Or we may adopt a sceptical attitude as to the reliability of the experimenter whose observations^ which threaten our system, we may exclude from science on the ground that they are insufficiently supported unscientific, or not objective, or even on the ground that the experimenter was a liar. (This is the sort of attitude which the physicist may sometimes quite rightly adopt towards alleged occult phenomena.) In the last resort we can always cast doubt on the acumen of the theoretician (for example if he does not believe, as does Dingier, that the theory of electricity will one day be derived from Newton’s theory of gravitation).
y
Thus, according to the conventionalist view, it is not possible to divide systems of theories into falsifiable and non-falsifiable ones; or rather, such a distinction will be ambiguous. As a
consequence, our criterion of falsifiability must turn out to be useless as a criterion of demarcation.
n Methodological Rules
These objections of an imaginary conventionalist seem to me incontestable, just like the conventionalist philosophy itself. I admit that my criterion of falsifiability does not lead to an unambiguous classification. Indeed, it is impossible to decide, by analysing its logical form, whether a system of statements is a conventional system of irrefutable implicit definitions, or whether it is a system which is empirical in my sense; that is, a refutable system., Yet this only shows that my criterion of demarcation cannot be applied immediately to a system of statements - a fact I have already pointed out [in selection 8, section n, and selection 9, section v]. The question whether a given system should as such be regarded as a conventionalist or an empirical one is therefore misconceived. Only with reference to the methods applied to a theoretical system is it at all possible to ask whether we are dealing with a conventionalist or an empirical theory. The only way to avoid conventionalism is by taking a decision: the decision not to apply its methods. We decide that if our system is threatened we will never save it by any kind of conventionalist stratagem. Thus we shall guard against exploiting the ever open possibility just
mentioned of ‘.. .attaining, for any chosen ... system, what is called its “correspondence with reality”’.
A clear appreciation of what may be gained (and lost) by conventionalist methods was expressed, a hundred years before Poincare, by Black who wrote: ‘A nice adaptation of conditions will make almost any hypothesis agree with the phenomena. This will please the imagination but does not advance our knowledge.’5
In order to formulate methodological rules which prevent the adoption of conventionalist stratagems, we should have to acquaint ourselves with the various forms these stratagems may take, so as to meet each with the appropriate anti-conventionalist countermove. Moreover we should agree that, whenever we find that a system has been rescued by a conventionalist stratagem, we shall test it afresh, and reject it, as circumstances may require.
The four main conventionalist stratagems have already been listed at the end of the previous section. The list makes no claim to completeness: it must be left to the investigator, especially in the fields of sociology and psychology (the physicist may hardly need the warning) to guard constantly against the temptation to employ new conventionalist stratagems - a temptation to which psychoanalysts, for example, often succumb.
As regards auxiliary hypotheses we propose to lay down the rule that only those are acceptable whose introduction does not diminish the degree of falsifiability or testability of the system in question, but, on the contrary, increases it.6 If the degree of falsifiability is increased, then introducing the hypothesis has actually strengthened the theory: the system now rules out more than it did previously: it prohibits more. We can also put it like this. The introduction of an auxiliary hypothesis should always be regarded as an attempt to construct a new system; and this new system should then always be judged on the issue of whether it would, if adopted, constitute a real advance in our knowledge of the world. An example of an auxiliary hypothesis which is eminently acceptable in this sense is Pauli’s exclusion principle. An example of an unsatisfactory auxiliary hypothesis would be the contraction hypothesis of Fitzgerald and Lorentz which had no falsifiable consequences but merely7 served to restore the agreement between theory and experiment - mainly the findings of Michelson and Morley. An advance was here achieved only by the
theory of relativity which predicted new consequences, new physical effects, and thereby opened up new possibilities for testing, and for falsifying, the theory. Our methodological rule may be qualified by the remark that we need not reject, as conventionalistic, every auxiliary hypothesis that fails to satisfy these standards. In particular, there are singular statements which do not really belong to the theoretical system at all. They are sometimes called ‘auxiliary hypotheses’, and although they are introduced to assist the theory, they are quite harmless. (An example would be the assumption that a certain observation or measurement which cannot be repeated may have been due to error. [See selection 11, section n.])
Changes in explicit definitions, whereby the concepts of an axiom system are given a meaning in terms of a system of lower level universality, are permissible if useful; but they must be regarded as modifications of the system, which thereafter has to be re-examined as if it were new. As regards undefined universal names, two possibilities must be distinguished. (l)There are some undefined concepts which only appear in statements of the highest level of universality, and whose use is established by the fact that we know in what logical relation other concepts stand to them. They can be eliminated in the course of deduction (an example is ‘energy’).8 (2) There are other undefined concepts which occur in statements of lower levels of universality also, and whose meaning is established by usage (e.g. ‘movement’, ‘mass point’, ‘position’). In connection with these, we shall forbid surreptitious alterations of usage, and otherwise proceed in conformity with our methodological decisions, as before.
As to the two remaining points (which concern the competence of the experimenter or theoretician) we shall adopt similar rules. Intersubjectively testable experiments are either to be accepted, or to be rejected in the light of counterexperiments. The bare appeal to logical derivations to be discovered in the future can be disregarded.
Only in the case of systems which would be falsifiable if treated in accordance with our rules of empirical method is there any need
to guard against conventionalist strategems. Let us assume that we have successfully banned these stratagems by our rules: we may now ask for a logical characterization of such falsifiable systems. We shall attempt to characterize the falsifiability of a theory by the logical relations holding between the theory and the class of basic statements.
The character of the singular statements which I call ‘basic statements’ will be discussed more fully [in the next selection], and also the question whether they, in their turn, are falsifiable. Here we shall assume that falsifiable basic statements exist. It should be borne in mind that when I speak of ‘basic statements’, I am not referring to a system of accepted statements. The system of basic statements, as I use the term, is to include, rather, all self-consistent singular statements of a certain logical form - all conceivable singular statements of fact, as it were. Thus the system of all basic statements will contain many statements which are mutually incompatible.
As a first attempt one might perhaps try calling a theory ‘empirical’ whenever singular statements can be deduced from it. This attempt fails, however, because in order to deduce singular statements from a theory, we always need other singular statements - the initial conditions that tell us what to substitute for the variables in the theory. As a second attempt, one might try calling a theory ‘empirical’ if singular statements are derivable with the help of other singular statements serving as initial conditions. But this will not do either; for even a non-empirical theory, for example a tautological one, would allow us to derive some singular statements from other singular statements. (According to the rules of logic we can for example say: from the conjunction of‘Twice two is four’ and ‘Here is a black raven’ there follows, among other things, ‘Here is a raven’.) It would not even be enough to demand that from the theory together with some initial conditions we should be able to deduce more than we could deduce from those initial conditions alone. This demand would indeed exclude tautological theories, but it would not exclude synthetic metaphysical statements. (For example from ‘Every occurrence has a cause’ and ‘A catastrophe is occurring here’, we can deduce ‘This
4
catastrophe has a cause’.)
In this way we are led to the demand that the theory should allow
us to deduce, roughly speaking, more empirical singular statements than we can deduce from the initial conditions alone.9 This means that we must base our definition upon a particular class of singular statements; and this is the purpose for which we need the basic statements. Seeing that it would not be very easy to say in detail how a complicated theoretical system helps in the deduction of singular or basic statements, I propose the following definition. A theory is to be called ‘empirical’ or ‘falsifiable’ if it divides the class of all possible basic statements unambiguously into the following two non-empty subclasses. First, the class of all those basic statements with which it is inconsistent (or which it rules out, or prohibits): we call this the class of the potential falsifiers of the theory; and secondly, the class of those basic statements which it does not contradict (or which it ‘permits’). We can put this more briefly by saying: a theory is falsifiable if the class of its potential falsifiers is not empty.
It may be added that a theory makes assertions only about its potential falsifiers. (It asserts their falsity.) About the ‘permitted’ basic statements it says nothing. In particular, it does not say that they are true.10
We must clearly distinguish between falsifiability and falsification. We have introduced falsifiability solely as a criterion for the empirical character of a system of statements. As to falsification, special rules must be introduced which will determine under what conditions a system is to be regarded as falsified.
We say that a theory is falsified only if we have accepted basic statements which contradict it. [See selection 9, section v.] This condition is necessary, but not sufficient; for non-reproducible single occurrences are of no significance to science. Thus a few stray basic statements contradicting a theory will hardly induce us to reject it as falsified. We shall take it as falsified only if we discover a reproducible effect which refutes the theory. In other words, we only accept the falsification if a low-level empirical hypothesis which describes such an effect is proposed and corroborated. This kind of hypothesis may be called a falsifying hypothesis. The requirement that the falsifying hypothesis must be
empirical, and so falsifiable, only means that it must stand in a certain logical relationship to possible basic statements; thus this requirement only concerns the logical form of the hypothesis. The rider that the hypothesis should be corroborated refers to tests which it ought to have passed - tests which confront it with accepted basic statements.11
Thus the basic statements play two different roles. On the one hand, we have used the system of all logically possible basic statements in order to obtain with its help the logical characterization for which we were looking - that of the form of empirical statements. On the other hand, the accepted basic statements are the basis for the corroboration of hypotheses. If accepted basic statements contradict a theory, then we take them as providing sufficient grounds for its falsification only if they corroborate a falsifying hypothesis at the same time.
We have now reduced the question of the falsifiability of theories to that of the falsifiability of those singular statements which I have called basic statements. But what kind of singular statements are these basic statements? How can they be falsified? To the practical research worker, these questions may be of little concern. But the obscurities and misunderstandings which surround the problem make it advisable to discuss it here in some detail.
The doctrine that the empirical sciences are reducible to sense perceptions, and thus to our experiences, is one which many accept as obvious beyond all question. However, this doctrine stands or falls with inductive logic, and is here rejected along with it. I do not wish to deny that there is a grain of truth in the view that mathematics and logic are based on thinking, and the factual sciences on sense perceptions. But what is true in this view has little bearing on the epistemological problem. And indeed, there is hardly a problem in epistemology which has suffered more severely from the confusion of psychology with logic than this problem of the basis of statements of experience.
The problem of the basis of experience has troubled few thinkers so deeply as Fries.1 He taught that, if the statements of science are not to be accepted dogmatically, we must be able to justify them. If we demand justification by reasoned argument, in the logical sense, then we are committed to the view that statements can be justified only by statements. The demand that all statements are to be logically justified (described by Fries as a ‘predilection for proofs’) is therefore bound to lead to an infinite regress. Now, if we wish to avoid the danger of dogmatism as well as an infinite regress, then it seems as if we could only have recourse to psychologism, i.e.
the doctrine that statements can be justified not only by statements but also by perceptual experience. Faced with this trilemma -dogmatism vs. infinite regress vs. psychologism - Fries, and with him almost all epistemologists who wished to account for our empirical knowledge, opted for psychologism. In sense experience, he taught, we have ‘immediate knowledge’:2 by this immediate knowledge, we may justify our ‘mediate knowledge’ -knowledge expressed in the symbolism of some language. And this mediate knowledge includes, of course, the statements of science.
Usually the problem is not explored as far as this. In the epistemologies of sensationalism and positivism it is taken for granted that empirical scientific statements ‘speak of our experiences’.3 For how could we ever reach any knowledge of facts if not through sense perception? Merely by taking thought a man cannot add an iota to his knowledge of the world of facts. Thus perceptual experience must be the sole ‘source of knowledge’ of all the empirical sciences. All we know about the world of facts must therefore be expressible in the form of statements about our experiences. Whether this table is red or blue can be found out only by consulting our sense experience. By the immediate feeling of conviction which it conveys, we can distinguish the true statement, the one whose terms agree with experience, from the false statement, whose terms do not agree with it. Science is merely an attempt to classify and describe this perceptual knowledge, these immediate experiences whose truth we cannot doubt; it is the
systematic presentation of our immediate convictions.
This doctrine founders in my opinion on the problems of induction and of universals. For we can utter no scientific statement that does not go far beyond what can be known with certainty ‘on the basis of immediate experience’. (This fact may be referred to as the ‘transcendence inherent in any description’.) Every description uses universal names (or symbols, or ideas); every statement has the character of a theory, of a hypothesis. The statement, ‘Here is a glass of water’ cannot be verified by any observational experience. The reason is that the universals which appear in it cannot be correlated with any specific sense experience. (An ‘immediate experience’ is only once ‘immediately given’; it is unique.) By the word ‘glass’, for example, we denote
physical bodies which exhibit a certain lawlike behaviour, and the same holds for the word ‘water’. Universals cannot be reduced to classes of experiences; they cannot be ‘constituted’.4
n The Objectivity of the Empirical Basis
I propose to look at science in a way which is slightly different from that favoured by the various psychologists schools: I wish to distinguish sharply between objective science on the one hand, and ‘our knowledge’ on the other.
I readily admit that only observation can give us ‘knowledge concerning facts’, and that we can (as Hahn says) ‘become aware of facts only by observation’. But this awareness, this knowledge of ours, does not justify or establish the truth of any statement. I do not believe, therefore, that the question which epistemology must ask is, ‘... on what does our knowledge rest? ... or more exactly, how can I, having had the experience S, justify my description of it, and defend it against doubt?’5 This will not do, even if we change the term ‘experience’ into ‘protocol sentence’. In my view, what epistemology has to ask is, rather: how do we test scientific statements by their deductive consequences? (Or, more generally: how can we best criticize our theories (our hypotheses, our guesses), rather than defend them against doubt? [See also selection 3, section hi.]) And what kind of consequences can we select for this purpose if they in their turn are to be intersubjectively testable?
By now, this kind of objective and non-psychological approach is pretty generally accepted where logical or tautological statements are concerned. Yet not so long ago it was held that logic was a science dealing with mental processes and their laws - the laws of our thought. On this view there was no other justification to be found for logic than the alleged fact that we just could not think in any other way. A logical inference seemed to be justified because it was experienced as a necessity of thought, as a feeling of being compelled to think along certain lines. In the field of logic, this kind of psychologism is now perhaps a thing of the past. Nobody would dream of j ustifying the validity of a logical inference, or of defending it against doubts, by writing beside it in the margin the
following protocol sentence. ‘Protocol: In checking this chain of inferences today, I experienced an acute feeling of conviction.’
The position is very different when we come to empirical statements of science. Here everybody believes that these are grounded on experiences such as perceptions; or in the formal mode of speech, on protocol sentences. Most people would see that any attempt to base logical statements on protocol sentences is a case of psychologism. But curiously enough, when it comes to empirical statements, the same kind of thing goes today by the name of ‘physicalisin’. Yet whether statements of logic are in question or statements of empirical science, I think the answer is the same: our knowledge, which may be described vaguely as a system of dispositions, and which may be of concern to psychology, may be in both cases linked with feelings of belief or of conviction: in the one case, perhaps, with the feeling of being compelled to think in a certain way; in the other with that of ‘perceptual assurance’. But all this interests only the psychologists. It does not even touch upon problems like those of the logical connections between scientific statements, which alone interest the epistemo-logist.
(There is a widespread belief that the statement ‘I see that this table here is white’, possesses some profound advantage over the statement ‘This table here is white’, from the point of view of epistemology. But from the point of view of evaluating its possible objective tests, the first statement, in speaking about me, does not appear more secure than the second statement, which speaks about the table here.)
There is only one way to make sure of the validity of a chain of logical reasoning. This is to put it in the form in which it is most easily testable: we break it up into many small steps, each easy to check by anybody who has learnt the mathematical or logical technique of transforming sentences. If after this anybody still raises doubts then we can only beg him to point out an error in the steps of the proof, or to think the matter over again. In the case of the empirical sciences, the situation is much the same. Any empirical scientific statement can be presented (by describing experimental arrangements, etc.) in such a way that anyone who has learnt the relevant technique can test it. If, as a result, he rejects the statement, then it will not satisfy us if he tells us all about his
feelings of doubt or about his feelings of conviction as to his perceptions. What he must do is to formulate an assertion which contradicts our own, and give us his instructions for testing it. If he fails to do this we can only ask him to take another and perhaps a more careful look at our experiment, and think again.
An assertion which owing to its logical form is not testable can at best operate, within science, as a stimulus: it can suggest a problem. In the field of logic and mathematics, this may be exemplified by Fermat’s problem, and in the field of natural history, say, by reports about seaserpents. In such cases science does not say that the reports are unfounded; that Fermat was in error or that all the records of observed seaserpents are lies. Instead, it suspends judgement.
Science can be viewed from various standpoints, not only from that of epistemology; for example, we can look at it as a biological or as a sociological phenomenon. As such it might be described as a tool, or an instrument, comparable perhaps to some of our industrial machinery. Science may be regarded as a means of production - as the last word in ‘roundabout production’.6 Even from this point of view science is no more closely connected with ‘our experience’ than other instruments or means of production. And even if we look at it as gratifying our intellectual needs, its connection with our experiences does not differ in principle from that of any other objective structure. Admittedly it is not incorrect to say that science is ‘... an instrument’ whose purpose is ‘... to predict from immediate or given experiences later experiences, and even as far as possible to control them’.7 But I do not think that this talk about experiences contributes to clarity. It has hardly more point than, say, the not incorrect characterization of an oil derrick by the assertion that its purpose is to give us certain experiences: not oil, but rather the sight and smell of oil; not money, but rather the feeling of having money.
m Basic Statements
It has already been briefly indicated what role the basic statements play within the epistemological theory I advocate. We need them in order to decide whether a theory is to be called falsifiable, i.e.
empirical. And we also need them for the corroboration of falsifying hypotheses, and thus for the falsification of theories. [See selection 10, sections hi and iv respectively.]
Basic statements must therefore satisfy the following conditions. (1) From a universal statement without initial conditions, no basic statement can be deduced.8 On the other hand, (2) a universal statement and a basic statement can contradict each other. Condition (2) can only be satisfied if it is possible to derive the negation of a basic statement from the theory which it contradicts. From this and condition (1) it follows that a basic statement must have a logical form such that its negation cannot be a basic statement in its turn.
There is a familiar example of statements whose logical form is different from that of their negations. These are universal statements and existential statements: they are negations of one another, and they differ in their logical form. Singular statements can be constructed in an analogous way. The statement: ‘There is a raven in the spacetime region ky may be said to be different in
n
its logical form - and not only in its linguistic form - from the statement ‘There is no raven in the spacetime region k\ A statement of the form ‘There is a so-and-so in the region k’ or ‘Such-and-such an event is occurring in the region ky may be called a ‘singular existential statement’ or a ‘singular there-is statement’. And the statement which results from negating it, i.e. ‘There is no so-and-so in the region k’ or ‘No event of such-and-such a kind is occurring in the region k\ may be called a ‘singular non-existence statement’, or a ‘singular there-is-not statement’.
We may now lay down the following rule concerning basic
s
statements: basic statements have the form of singular existential statements. This rule means that basic statements will satisfy condition (1), since a singular existential statement can never be deduced from a strictly universal statement, i.e. from a strict non-existence statement. They will also satisfy condition (2), as can be seen from the fact that from every singular existential statement a purely existential statement can be derived simply by omitting any reference to any individual spacetime region; and as we have seen, a purely existential statement may indeed contradict a theory.
It should be noticed that the conjunction of two basic
statements, d and r, which do not contradict each other, is in turn a basic statement. Sometimes we may even obtain a basic statement by joining one basic statement to another statement which is not basic. For example, we may form the conjunction of the basic statement, r, ‘There is a pointer at the place k’ with the singular non-existence statement p, ‘There is no pointer in motion at the place k\ For clearly, the conjunction r-p (‘r-and-non-p’) of the two statements is equivalent to the singular existential statement ‘There is a pointer at rest at the place k\ This has the consequence that, if we are given a theory t and the initial conditions r, from which we deduce the prediction p, then the statement r-p will be a falsifier of the theory, and so a basic statement. (On the other hand, the conditional statement ‘r-»p’ i.e. ‘If r then p’, is no more basic than the negation p, since it is equivalent to the negation of a basic statement, viz. to the negation of r-p.)
These are the formal requirements for basic statements; they are satisfied by all singular existential statements. In addition to these, a basic statement must also satisfy a material requirement - a requirement concerning the event which, as the basic statement tells us, is occurring at the place k. This event must be an ‘observable’ event; that is to say, basic statements must be testable, intersubjectively, by ‘observation’. Since they are singular statements, this requirement can of course only refer to observers who are suitably placed in space and time (a point which I shall not elaborate).
No doubt it will now seem as though in demanding observability, I have, after all, allowed psychologism to slip back quietly into my theory. But this is not so. Admittedly, it is possible to interpret the concept of an observable event in a psychologistic sense. But I am using it in such a sense that it might just as well be replaced by ‘an event involving position and movement of macroscopic physical bodies’. Or we might lay it down, more precisely, that every basic statement either must be itself a statement about relative positions of physical bodies, or must be equivalent to some basic statement of this ‘mechanistic’ or ‘materialistic’ kind. (That this stipulation is practicable is connected with the fact that a theory which is intersubjectively testable will also be intersen-sually9 testable. This is to say that tests involving the perception of one of our senses can, in principle, be replaced by tests involving other senses.) Thus the charge that, in appealing to observability, I have stealthily re-admitted psychologism would have no more force than the charge that I have admitted mechanism or materialism. This shows that my theory is really quite neutral and that neither of these labels should be pinned to it. I say all this only so as to save the term ‘observable’, as I use it, from the stigma of psychologism. (Observations and perceptions may be psychological but observability is not.) I have no intention of defining the term ‘observable’ or ‘observable event’, though I am quite ready to elucidate it by means of either psychologistic or mechanistic examples. I think that it should be introduced as an undefined term which becomes sufficiently precise in use: as a primitive concept whose use the epistemologist has to learn, much as he has to learn the use of the term ‘symbol’, or as the physicist has to learn the use of the term ‘mass point’.
Basic statements are therefore - in the material mode of speech - statements asserting that an observable event is occurring in a certain individual region of space and time.
iv The Relativity of Basic Statements. Resolution of Fries’s
Trilemma
Every test of a theory, whether resulting in its corroboration or falsification, must stop at some basic statement or other which we decide to accept. If we do not come to any decision, and do not accept some basic statement or other, then the test will have led nowhere. But considered from a logical point of view, the situation is never such that it compels us to stop at this particular basic statement rather than at that, or else give up the test altogether. For any basic statement can again in its turn be subjected to tests, using as a touchstone any of the basic statements which can be deduced from it with the help of some theory, either the one under test, or another. This procedure has no natural end. Thus if the test is to lead us anywhere, nothing remains but to stop at some point or other and say that we are satisfied, for the time being.
It is fairly easy to see that we arrive in this way at a procedure according to which we stop only at a kind of statement that is especially easy to test. For it means that we are stopping at statements about whose acceptance or rejection the various investigators are likely to reach agreement. And if they do not agree, they will simply continue with the tests, or else start them all over again. If this too leads to no result, then we might say that the statements in question were not intersubjectively testable, or that we were not, after all, dealing with observable events. If some day it should no longer be possible for scientific observers to reach agreement about basic statements this would amount to a failure of language as a means of universal communication. It would amount to a new ‘Babel of Tongues’: scientific discovery would be reduced to absurdity. In this new Babel, the soaring edifice of science would soon lie in ruins.
Just as a logical proof has reached a satisfactory shape when the difficult work is over, and everything can be easily checked, so, after science has done its work of deduction or explanation, we stop at basic statements which are easily testable. Statements about personal experiences - i.e. protocol sentences - are clearly not of this kind; thus they will not be very suitable to serve as statements at which we stop. We do of course make use of records or protocols, such as certificates of tests issued by a department of scientific and industrial research. These, if the need arises, can be re-examined. Thus it may become necessary, for example, to test the reaction times of the experts who carry out the tests (i.e. to determine their personal equations). But in general, and especially ‘.. .in critical cases’ we do stop at easily testable statements, and not, as Carnap recommends, at perception or protocol sentences; i.e. we do not ‘.. .stop just at these ... because the intersubjective testing of statements about perceptions ... is relatively complicated and
difficult’.10
What is our position now in regard to Fries’s trilemma, the choice between dogmatism, infinite regress, and psychologism? [See section i above.] The basic statements at which we stop, which we decide to accept as satisfactory, and as sufficiently tested, have admittedly the character of dogmas, but only in so far as we may desist from justifying them by further arguments (or by further tests). But this kind of dogmatism is innocuous since, should the need arise, these statements can easily be tested further. I admit that this too makes the chain of deduction in principle infinite. But this kind of ‘infinite regress’ is also innocuous since in our theory there is no question of trying to prove any statements by means of
it. And finally, as to psychologism: I admit, again, that the decision to accept a basic statement, and to be satisfied with it, is causally connected with our experiences - especially with our perceptual experiences. But we do not attempt to justify basic statements by these experiences. Experiences can motivate a decision, perhaps decisively, and hence an acceptance or a rejection of a statement, but a basic statement cannot be justified by them - no more than by thumping the table.11