A Note on Popper’s Demarcation line between science and non-science

This is in response to Mura Nava’s request for “a short-winded view of the sci / non-sci” distinction, after I’d suggested that Raphael Berthele’s  article was long-winded and confused.

Popper says falsifiability is the hallmark of a scientific theory, and allows us to make a demarcation line between science and non-science: if a theory doesn’t make predictions that can be falsified, it’s not scientific.  According to such a demarcation, astronomy is scientific and astrology is not, since although there are millions of examples of true predictions made by astrologers, astrologers deny that false predictions constitute a challenge to their theory.

Popper insists that in scientific investigation we start with problems, not with empirical observations, and that we then leap to a solution of the problem we have identified – in any way we like. This second stage is crucial to an understanding of Popper’s epistemology: when we are at the stage of coming up with explanations, with theories or hypotheses, then, in a very real sense, anything goes. Inspiration can come from lowering yourself into a bath of water, being hit on the head by an apple, or by imbibing narcotics. It’s at the next stage of the theory-building process that empirical observation comes in, and, according to Popper, its role is not to provide data that confirm the theory, but rather to find data that test it.

Empirical observations should be carried out in attempts to falsify the theory in question. So, for example, if we claim that all swans are white, we should search high and low for a non-white swan, or if we claim that the earth rotates towartds the east, we should try and observe the sun rising in the West, etc. The implication is that the theory has to be formulated in such a way that empirical tests can be carried out: there must be, at least in principle, some empirical observation that could clash with the explanations and predictions that the theory offers. If the theory survives repeated attempts to falsify it, then we can hold on to it tentatively, even though we’ll never know for certain that it’s true. The bolder the theory (i.e. the more it exposes itself to testing, the more wide-ranging its consequences, the riskier it is) the better. If the theory doesn’t stand up to the tests, if it’s falsified, then we need to re-define the problem, come up with an improved solution, a better theory, and then test it again to see if it stands up to empirical tests more successfully. These successive cycles are an indication of the growth of knowledge (Popper, 1974).

Popper (1974: 105-106) gives the following diagram to explain his view:

P1 -> TT -> EE -> P2

P = problem    TT = tentative theory    EE = Error Elimination through empirical experiments which test the theory

We begin with a problem (P1), which we should articulate as well as possible.  We then propose a tentative theory (TT), that tries to explain the problem. We can arrive at this theory in any way we choose, but we must formulate it in such a way that it leaves itself open to empirical tests. The empirical tests and experiments (EE) that we devise for the theory have the aim of trying to falsify it. These experiments usually generate further problems (P2) because they contradict other experimental findings, or they clash with the theory’s predictions, or they cause us to widen our questions. The new problems give rise to a new tentative theory and the need for more empirical testing.

Popper thus gives empirical experiments and observation a completely different role to the one given to them by the empiricists and positivists: their job now is to test a theory, not to prove it, and since this is a deductive approach, it escapes Hume’s famous problem of induction. Popper takes advantage of the asymmetry between verification and falsification: while no number of empirical observations can ever prove a theory is true, just one such observation can prove that it is false. All you need is to find one black swan and the theory “All swans are white” is disproved.

Popper claimed that his research methodology, and the epistemology that informs it, is rationalist because it insists that we arrive at knowledge of the world, and in particular, that we build scientific theories to explain aspects of our world, by using our minds creatively. However, Popper also argues that we need to test these a priori ideas empirically. Hence, the two central premises of Popper’s argument are:

  • Knowledge advances by trying to solve problems in a rational way.
  • The role of empirical data is to test theories.


It’s worth noting here that “positivism” is not a synonym for a scientific method which relies on logic and empirical tests. It was a philosophical movement culminating in the writings of the Vienna Circle, and is, in my opinion, a good example of philosophers stubbornly marching up a blind alley. It is a fundamentally mistaken project, as Popper has, I believe, shown, and as Wittgenstein himself recognised in his later work (see Wittgenstein, 1953). Those critics of mainstream SLA research who label their opponents “positivists”, or who argue against “positivist science”, are either ignorant of the history of positivism or are making a straw man case which no present-day researcher in the field of SLA adopting a rationalist position need defend.

A clear distinction must also be made between empiricism as a philosophical position (a position taken by Skinner, for example and currently flirted with by emergentists) and empirical evidence – observations that can be measured and checked. Popper rejects the strict epistemolgical claims of empiricism, and claims instead that scientific theories must have some empirical content and be open to empirical tests.


Popper, K. R. (1959) The Logic of Scientific Discovery.  London: Hutchinson.

Popper, K. R. (1963) Conjectures and Refutations.  London: Hutchinson.

Popper, K. R. (1972) Objective Knowledge.  Oxford: Oxford University Press.

Popper, K. (1974) Replies to Critics in P.A. Schilpp (ed.), The Philosophy of Karl Popper. Open Court, La Salle, III.

Wittgenstein, L. (1953) Philosophical Investigations. Translated by G:E.M. Anscombe. Oxford: Basil Blackwell.