Tag: falsifiability

Strict logical analysis only gets you nowhere

I am quite ready to admit that there is a need for a purely logical analysis of theories, for an analysis which takes no account of how they change and develop. But this kind of analysis does not elucidate those aspects of the empirical sciences which I, for one, so highly prize. A system such as classical mechanics may be ‘scientific’ to any degree you like; but those who uphold it dogmatically—believing, perhaps, that it is their business to defend such a successful system against criticism as long as it is not conclusively disproved—are adopting the very reverse of that critical attitude which in my view is the proper one for the scientist. In point of fact, no conclusive disproof of a theory can ever be pro­duced; for it is always possible to say that the experimental results are not reliable, or that the discrepancies which are asserted to exist between the experimental results and the theory are only apparent and that they will disappear with the advance of our understanding. (In the struggle against Einstein, both these arguments were often used in support of Newtonian mechanics, and similar arguments abound in the field of the social sciences.) If you insist on strict proof (or strict disproof) in the empirical sciences, you will never benefit from experience, and never learn from it how wrong you are.

If therefore we characterize empirical science merely by the formal or logical structure of its statements, we shall not be able to exclude from it that prevalent form of metaphysics which results from elevating an obsolete scientific theory into an incontrovertible truth.

Such are my reasons for proposing that empirical science should be characterized by its methods: by our manner of dealing with scientific systems: by what we do with them and what we do to them. Thus I shall try to establish the rules, or if you will the norms, by which the scientist is guided when he is engaged in research or in discovery, in the sense here understood. [28-9]

Evading falsification

A third objection may seem more serious. It might be said that even if the asymmetry is admitted, it is still impossible, for various reasons, that any theoretical system should ever be conclusively falsified. For it is always possible to find some way of evading falsification, for example by introducing ad hoc an auxiliary hypothesis, or by changing ad hoc a defini­tion. It is even possible without logical inconsistency to adopt the position of simply refusing to acknowledge any falsi­fying experience whatsoever. Admittedly, scientists do not usually proceed in this way, but logically such procedure is possible; and this fact, it might be claimed, makes the logical value of my proposed criterion of demarcation dubious, to say the least.

I must admit the justice of this criticism; but I need not therefore withdraw my proposal to adopt falsifiability as a criterion of demarcation. For I am going to propose that the empirical method shall be characterized as a method that excludes precisely those ways of evading falsification which, as my imaginary critic rightly insists, are logically possible. Accord­ing to my proposal, what characterizes the empirical method is its manner of exposing to falsification, in every conceiv­able way, the system to be tested. Its aim is not to save the lives of untenable systems but, on the contrary, to select the one which is by comparison the fittest, by exposing them all to the fiercest struggle for survival. [19-20]

Singled out by experience

Now in my view there is no such thing as induction. Thus inference to theories, from singular statements which are ‘veri­fied by experience’ (whatever that may mean), is logically inadmissible. Theories are, therefore, never empirically veri­fiable. If we wish to avoid the positivist’s mistake of eliminating, by our criterion of demarcation, the theoretical systems of natural science, then we must choose a criterion which allows us to admit to the domain of empirical science even statements which cannot be verified.

But I shall certainly admit a system as empirical or scientific only if it is capable of being tested by experience. These considerations suggest that not the verifiability but the falsifiability of a system is to be taken as a criterion of demar­cation. In other words: I shall not require of a scientific system that it shall be capable of being singled out, once and for all, in a positive sense; but I shall require that its logical form shall be such that it can be singled out, by means of empi­rical tests, in a negative sense: it must be possible for an empirical scientific system to be refuted by experience. [18]

Experience as a method

[W]e may distinguish three requirements which our empirical theoretical system will have to satisfy. First, it must be syn­thetic, so that it may represent a non-contradictory, a possible world. Secondly, it must satisfy the criterion of demar­cation, i.e. it must not be metaphysical, but must represent a world of possible experience. Thirdly, it must be a system distinguished in some way from other such systems as the one which represents our world of experience.

But how is the system that represents our world of experience to be distinguished? The answer is: by the fact that it has been submitted to tests, and has stood up to tests. This means that it is to be distinguished by applying to it that deduc­tive method which it is my aim to analyse, and to describe.

‘Experience’, on this view, appears as a distinctive method whereby one theoretical system may be distinguished from others; so that empirical science seems to be characterized not only by its logical form but, in addition, by its distinctive method. [16-7]

Uninteresting theories

The a posteriori evaluation of a theory depends entirely upon the way it has stood up to severe and ingenious tests. But severe tests, in their turn, presuppose a high degree of a priori testability or content. Thus the a posteriori evaluation of a theory depends laregely upon its a priori value: theories which are a priori uninteresting—of little content—need not be tested because their low degree of testability excludes a priori the possibility that they may be subjected to really significant and interesting tests.

On the other hand, highly testable theories are interesting and important even if they fail to pass their test; we can learn immensely from their failure. Their failure may be fruitful, for it may actually suggest how to construct a better theory. [143-4]

Destroying authoritarianism in science

For there never was a more successful theory, or a better tested theory, than Newton’s theory of gravity. It succeeded in explaining both terrestrial and celestial mechanics. It was most severely tested in both fields for centuries. The great physicist and mathematician Henri Poincaré believed not only that it was true – this of course was everybodys belief – but that it was true by definition, and that it would therefore remain the invariable basis of physics to the end of man’s search for truth. And Poincaré believed this in spite of the fact that he actually anticipated – or that he came very close to anticipating – Einstein’s special theory of relativity. I mention this in order to illustrate the tremendous authority of Newton’s theory down to the very last.

Now the question whether or not Einstein’s theory of gravity is an improvement upon Newton’s, as most physicists think it is, may be left open. But the mere fact that there was now an alternative theory which explained everything that Newton could explain and, in addition, many more things, and which passed at least one of the crucial tests that Newton’s theory seemed to fail, destroyed the unique place held by Newton’s theory in its field. Newton’s theory was thus reduced to the status of an excellent and successful conjecture, a hypothesis competing with others, and one whose acceptability was an open question. Einstein’s theory thus destroyed the authority of Newton’s, and with it some­thing of even greater importance – authoritarianism in science. [91]

Testing a theory means trying to fault it

This criterion of demarcation between empirical and non-empirical theories I have also called the criterion of falsifia­bility or the criterion of refutability. It does not imply that irrefutable theories are false. Nor does it imply that they are meaningless. But it does imply that, as long as we cannot describe what a possible refutation of a certain theory would be like, that theory may be regarded as lying outside the field of empirical science.

The criterion of refutability or falsifiability may also be called the criterion of testability. For testing a theory, like testing a piece of machinery, means trying to fault it. Thus a theory that we know in advance cannot possibly be faulted or refuted is not testable. [88]

The mental process of critical rationalism

It is the fashion of the present time to disparage negative logic—that which points out weaknesses in theory or errors in practice, without establishing positive truths. Such negative criticism would indeed be poor enough as an ultimate re­sult; but as a means to attaining any positive knowledge or conviction worthy the name, it cannot be valued too highly; and until people are again systematically trained to it, there will be few great thinkers, and a low general average of intellect, in any but the mathematical and physical departments of speculation. On any other subject no one’s opinions deserve the name of knowledge, except so far as he has either had forced upon him by others, or gone through of him­self, the same mental process which would have been required of him in carrying on an active controversy with oppo­nents. That, therefore, which when absent, it is so indispensable, but so difficult, to create, how worse than absurd is it to forego, when spontaneously offering itself! If there are any persons who contest a received opinion, or who will do so if law or opinion will let them, let us thank them for it, open our minds to listen to them, and rejoice that there is some one to do for us what we otherwise ought, if we have any regard for either the certainty or the vitality of our convictions, to do with much greater labour for ourselves. [ch. II, 57-8]

The weakest part of your opinion

A person who derives all his instruction from teachers or books, even if he escape the besetting temptation of con­tenting himself with cram, is under no compulsion to hear both sides; accordingly it is far from a frequent accomplish­ment, even among thinkers, to know both sides; and the weakest part of what everybody says in defence of his opinion, is what he intends as a reply to antagonists. [ch. II, 57]

Veiled induction: explanations based on evidence

Darwinian evolution is the most robust of scientific theories, but many people find it easier and more reassuring to believe that something so intricately constructed as life on Earth must have had a designer. It is hard for us to accept that erverything around us, from the beautry of the butterfly to the complexity of the human eye, could have evolved simply through chance and the pressure to survive. But the scientific evidence that evolution by natural selection has occurred is unassailable.

The scientific method, which is today practised in all corners of the globe, builds on explanations based on evidence, and when new evidence emerges that does not fit with the model, the explanation must change. That is how science moves on. [15]