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In his Logik der Forschung Popper writes that

basic statements are not justifiable by our immediate experiences, but are … accepted by an act, a free decision (LSD, 109)

The SEP entry on Popper comments as follows:

this itself seems to be a refined form of conventionalism—it implies that it is almost entirely an arbitrary matter whether it is accepted that a potential falsifier is an actual one, and consequently that the falsification of a theory is itself the function of a ‘free’ and arbitrary act.

I wonder: Why can't Popper claim that these basic statements ("present-tense observation statements about sense-data") are known to likely be true by, accepted by appeal to, logical inference, either deductive or inductive, from sense data?

How do we accept basic statements in a non-arbitrary way?

And, if we can't, it surely must matter that falsification and scientific belief is then arbitrary: doesn't it defeat his answer to the demarcation problem, meaning all scientific belief (vitalism is false) is irrational?

  • i guess that empirical deduction runs counter to the rest of his philosophy, and that induction [justifying basic statements] would mean that falsifying a theory is inductive. but what are either terminal to falsificationism? – user6917 Aug 13 '14 at 17:22
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    "Basic statements are true by logic[al] inference" - inference from what? Could you explain your idea a bit further? As it stands, it's a bit difficult to grasp what you're after. – DBK Aug 13 '14 at 20:22
  • popper says that our acceptance of basic statements is a matter of convention nothing more; and this does mean that falsification is likewise an arbitrary "free choice". why is he impelled into the claim that science is irrational, when he could just claim that basic statements are justified by our sense data? – user6917 Aug 15 '14 at 4:56
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You ask:

Why can't Popper claim that these basic statements ("present-tense observation statements about sense-data") are known to be true by logic inference, either deductive or inductive?

First, you have mis-stated the definition of a basic statement (Section 7 of LScD [Logic of Scientific Discovery]):

What I call a ‘basic statement’or a ‘basic proposition’ is a statement which can serve as a premise in an empirical falsification; in brief, a statement of a singular fact.

And to clarify further in note *2 to Section 4:

When I invented the new name ‘basic statement’ (or ‘basic proposition’; see below, sections 7 and 28) I did so only because I needed a term not burdened with the connotation of a perception statement. But unfortunately it was soon adopted by others, and used to convey precisely the kind of meaning which I wished to avoid.

A basic statement is not a statement about perception. It is a statement that some specific observable thing happened in some specific region of space and time. Further clarification (LScD, Section 28):

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 must either be itself a statement about relative positions of physical bodies, or that it must be equivalent to some basic statement of this ‘mechanistic’ or ‘materialistic’ kind.

No statement of any kind can be justified by induction because induction is impossible (Section 1 of LScD):

For the principle of induction must be a universal statement in its turn. Thus if we try to regard its truth as known from experience, then the very same problems which occasioned its introduction will arise all over again. To justify it, we should have to employ inductive inferences; and to justify these we should have to assume an inductive principle of a higher order; and so on. Thus the attempt to base the principle of induction on experience breaks down, since it must lead to an infinite regress.

In the last two paragraphs of Section 25 of LScD, Popper writes:

In the epistemologies of sensationalism and positivism it is taken for granted that empirical scientific statements ‘speak of our experiences’. 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 law-like behaviour, and the same holds for the word ‘water’. Universals cannot be reduced to classes of experiences; they cannot be ‘constituted’.

To understand Popper better you should read what he writes and take it literally. Don't assume that he can't really mean what he's writing. He does mean what he writes.

The Stanford Encyclopedia entry isn't worth much since it doesn't explain Popper's anti-justificationism (see Chapter I of "Realism and the Aim of Science"): justification (showing an idea is true or probably true) is impossible, undesirable and unnecessary. Debating about whether Popper justified this or that is irrelevant without a reply to his criticism of justificationism.

  • "A basic statement is not a statement about perception. It is a statement that some specific observable thing happened in some specific region of space and time." isn't that what i said "present-tense observation statements about sense-data" i nowhere mention perception, only observation. what sort of observational statements do not involve reference to sense data? – user6917 Aug 15 '14 at 2:20
  • should read: what sort of ob. statement cannot include reference to sense data? i didn't mean to say that all basic statements contain ineliminable reference to sense data. the definition is from SEP – user6917 Aug 15 '14 at 5:05
  • If the reference to sense data is eliminable then it should be eliminated. There's lots of stuff you can't sense that you can measure. For example, you can't see neutrinos or sense them in any other way but measurements of the number of neutrinos are about neutrinos, not about numbers recorded in a computer attached to a neutrino detector. – alanf Aug 15 '14 at 10:34
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    @another_name The term 'water' is a name that applies to all water everywhere in space and time. This implicitly assumes that some theory about water is true everywhere in space and time including the specific basic statement under consideration. There is no way of justifying that assumption so basic statements can't be justified. – alanf Apr 17 at 8:55
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    Basic statements are unjustified conjectures. You have to make a decision about whether to accept an alleged basic statement or not, but that decision is not arbitrary since you can and should control such decisions by criticism. For example, you can criticise bad observations through a telescope by explaining that optics implies the telescope was badly adjusted or whatever. – alanf Apr 17 at 12:11
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Basic statements are not justified by our immediate experiences ... but by an act, a free decision.

A free decision does not mean an arbitrary acceptance of a hypothesis, an observation or a deduction. Popper is merely acknowledging here that man is free and free to make decisions.

  • so how is the decision made non-arbitrarily? – another_name Apr 17 at 11:52
  • @another_name: Why not ask it to the community? That’s what is there for. You might want to explain where and why you are having trouble understanding the concept. – Mozibur Ullah Apr 17 at 11:59
  • that's what i was getting at in the question. i don't understand the concept because i have no idea how the decision is made non-arbitrarily. what is it based on if not arbitrary? ps i don't think popper is merely saying we're free to make free decisions (?) else it being a free decision wouldn't be a very cogent criticism of his ideas! – another_name Apr 17 at 12:02
  • @Another_name: when you walk do you use your legs ‘arbitrarily’. No, of course not. You put one foot in front of another, and walk. Similar principle. – Mozibur Ullah Apr 17 at 12:12
  • yes, but i don't think that's what is being said in the SEP article in referring to the act as free. why do they (the SEP) say it's "free and arbitrary" if the freedom of the decision has nothing to do with arbitrariness? why is it arbitray? – another_name Apr 17 at 12:33
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One answer could be that his is a conventionalism (as the SEP says), one that means that the decision is free and arbitrary (as the SEP says), but is free and arbitrary only for specific people, experts.

Then accepting or rejecting a scientific theory need not really be irrational, even if doing so is not a valid deductive argument.

  • i think this is actually a fair answer that takes seriously the claim that it is arbitrary, and then shows why / how that might not matter. obviously i haven't shown why it's arbitrary, but it does address one half of the question – another_name Apr 18 at 2:48

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