The falsifying mode of inference here referred to—the way in which the falsification of a conclusion entails the falsification of the system from which it is derived—is the *modus tollens* of classical logic. It may be described as follows:

Let *p* be a conclusion of a system *t* of statements which may consist of theories and initial conditions (for the sake of simplicity I will not distinguish between them). We may then symbolize the relation of derivability (analytical implication) of *p* from *t* by ‘*t ➙ p*’ which may be read: ‘*p* follows from *t *’. Assume *p* to be false, which we may write ‘*p*’, to be read ‘not-*p*’. Given the relation of deducibility, *t ➙ p*, and the assumption *p*, we can then infer *t * (read ‘not-*t *’); that is, we regard *t* as falsified. If we denote the conjunction (simultaneous assertion) of two statements by putting a point between the symbols standing for them, we may also write the falsifying inference thus: ((*t ➙ p*).*p*) ➙ *t *, or in words: ‘If *p* is derivable from *t*, and if *p* is false, then *t* also is false’.

By means of this mode of inference we falsify *the whole system* (the theory as well as the initial conditions) which was required for the deduction of the statement *p*, *i.e.* of the falsified statement. Thus it cannot be asserted of any one statement of the system that it is, or is not, specifically upset by the falsification. Only if *p* is *independent* of some part of the system can we say that this part is not involved in the falsification.* With this is connected the following possibility: we may, in some cases, perhaps in consideration of the *levels of universality*, attribute the falsification to some definite hypothesis—for instance to a newly introduced hypothesis. This may happen if a well-corroborated theory, and one which continues to be further corroborated, has been deductively explained by a new hypothesis of a higher level. The attempt will have to be made to test this new hypothesis by means of some of its consequences which have not yet been tested. If any of these are falsified, then we may well attribute the falsification to the new hypothesis alone. We shall then seek, in its stead, other high-level generalizations, but we shall not feel obliged to regard the old system, of lesser generality, as having been falsified.

* Thus we cannot at first know which among the various statements of the remaining sub-system *t ′* (of which *p* is not independent) we are to blame for the falsity of *p*; which of these statements we have to alter, and which we should retain. (I am not here discussing interchangeable statements.) It is often only the scientific instinct of the investigator (influenced, of course, by the results of testing and re-testing) that makes him guess which statements of *t ′* he should regard as innocuous, and which he should regard as being in need of modification. Yet it is worth remembering that it is often the modification of what we are inclined to regard as obviously innocuous (because of its complete agreement with our normal habits of thought) which may produce a decisive advance. A notable example of this is Einstein’s modification of the concept of simultaneity. [55-6]

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