Karl Popper’s ‘constitution of science’ (1)

‘The chess players’ (1876) by Thomas Eakins. Photo: Wikimedia Commons

In spite of the title of his most important book (The Logic of Scientific Discovery -LSD-, published originally in German as The Logik der Forschung, in 1935)1, Karl Popper’s fundamental claim about the rules of science is that they are not a matter of logic, in the sense that they cannot be derived from logical axioms, nor are something like ‘applied logic’. According to Popper, the logic of scientific discovery is a ‘logic’, at most, in the sense in which we can talk about ‘the logic of chess’:

Methodological rules are here regarded as conventions. They might be described as the rules of the game of empirical science. They differ from the rules of pure logic rather as do the rules of chess, which few would regard as part of pure logic: seeing that the rules of pure logic govern transformations of linguistic formulae, the result of an inquiry into the rules of chess could perhaps be entitled ‘The Logic of Chess’, but hardly ‘Logic’ pure and simple. (Similarly, the result of an inquiry into the rules of the game of science—that is, of scientific discovery— may be entitled ‘The Logic of Scientific Discovery’.) [All quotes are taken verbatim from LSD].

Conventions can be justified, argued in favour or against, but they cannot be ‘proved’ as logical or mathematical theorems, nor can they be ‘demonstrated’ or ‘confirmed’ as some (other than Popperians) think the laws of empirical science can. There is an indelible conventional element in the choice of the rules of any practice: one might decide not to ‘play’ the game according to those rules, though in that case probably she will not be playing that game, but a different one. Popper, hence, responds to the different forms of conventionalism (i.e., philosophical understandings of science according to which the truth of scientific claims is ‘conventional’), not by proving that conventionalism is self-consistent, but by admitting that science contains an indispensable conventional component built into it; this component, however, is not the truth (or the accepting-as-true) of the theories and laws scientists arrive, but the choice of the rules according to which scientists carry out the process leading them. Nevertheless, if we want this choice not to be purely arbitrary, it is necessary that we can engage in a rational discussion about the reasons why it could be better to play that game according to those rules, instead of according to others, or instead of a very different game. And these reasons are necessarily related to whether the proposed rules will actually lead us to satisfy the goals the game is intended to deliver. So, before studying what the rules of science should be, we need to have an idea of why we want to do science, instead of some different kind of activity. Just a few paragraphs after the text just quoted, Popper clearly formulates the goal he has in mind:

the rules are constructed with the aim of ensuring the applicability of our criterion of demarcation.

Popper’s criterion of demarcation, of course, is falsifiability: “it must be possible for an empirical scientific system to be refuted by experience”; or, a little bit more explicitly:

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.

Hence, the rules of science, according to Popper, must be ‘designed’ in order that all ‘scientific systems’ (theories? models? hypotheses? ‘theoretical frameworks’?) are, not only falsifiable through empirical testing, but also as much falsifiable as possible (as it is clear from rule R5 below, and from Popper’s insistence on ‘degrees of testability’ through the book, and especially on chapter 6). Before starting to discuss to what extent this is a sufficient goal to build on it a reasonable system of methodological rules, let’s see what norms had Popper in mind. One discouraging aspect of The Logic of Scientific Discovery from a contemporary point of view is that, once that its main goal of offering a defence of a particular view of the norms of science has stated, the book does not proceed to systematically formulate such rules but enters soon into some discussions characteristic of the philosophical concerns of the time, and of other problems suggested or illuminated by Popper’s approach. Of course, this is not to diminish the value of the book, which cannot be but a child of the circumstances. This lack of systematicity has led some commentators to pick and gather the rules that Popper actually formulate, though they are scattered more or less randomly through the text of the book. I will present here just the most important ones, starting by what Popper calls the ‘supreme rule’ (the enumeration is based on Ian Jarvie (2001)2):

(RS) The other rules of scientific procedure must be designed in such a way that they do not protect any statement in science against falsification.

(R1) The game of science is, in principle, without end. He who decides one day that scientific statements do not call for any further test, and that they can be regarded as finally verified, retires from the game.

(R2) Once a hypothesis has been proposed and tested, and has proved its mettle, it may not be allowed to drop out without ‘good reason’.

(R3) We are not to abandon the search for universal laws and for a coherent theoretical system, nor ever give up our attempts to explain causally any kind of event we can describe.

(R4) (Don’t) use undefined concepts as if they were implicitly defined.

(R5) Only those (auxiliary hypotheses) are acceptable whose introduction does not diminish the degree of falsifiability or testability of the system in question, but, on the contrary, increases it.

(R7) Inter-subjectively testable experiments are either to be accepted, or to be rejected in the light of counter-experiments.

(R9) After having produced some criticism of a rival tehory, we should always make a serious attempt to apply this criticism to our own theory.

(R10) We should not accept stray basic statements –i.e., logically disconnected ones- but … we should accept basic statements in the course of testing theories; or raising searching questions about these theories, to be answered by the acceptance of basic statements.

(R11) Those theories should be given preference which can be most severely tested… (i.e., favour) theories with the highest possible empirical content.

The complete list of these rules is what Jarvie has called Popper’s ‘Proto-constitution of science’. I shall devote the next entry to discuss several important problems I think can be recognized in this ‘constitution’, its evident lack of systematicity not being one of them.


  1. Popper, K. R., 1935/2002, The Logic of Scientific Discovery, London: Routledge.
  2. Jarvie, I., 2001, The Republic of Science. The Emergence of Popper’s Social View of Science, 1935-1945, Amsterdam: Rodopi.

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