Like everyone else I read Ayer’s Language, Truth and Logic as a teenager and, like many people of a scientific bent, I loved it. The Logical Positivism that it espoused can be summarised as the claim that knowledge is of two types: (1) logical reasoning from axioms, such as used by mathematics; and (2) claims about the universe that can (in principle) be verified empirically. Anything else — such as metaphysics — is literally meaningless.
Logical Positivism is generally held to have been refuted (following criticisms from notables such as Quine, Popper and others), and as stated in its original form that is a fair assessment. However, its general thrust can be defended as sound. Indeed, Logical Positivism was a forerunner of what today gets called scientism, and interpreting it as scientism it is very much alive.
First, a defender of scientism would subsume the first type of knowledge, the “logical reasoning from axioms”, into the knowledge that derives from empiricism and can be empirically verified. Afterall, the reason that we adopt our basic axioms of logic and mathematics is because they work — they give results that apply to our universe. Where else would we have got them from?
Philosophers suggest an alternative to empirical knowledge, namely knowledge from intuition. However, our brains are very much products of our empirical, material universe. Our adult brains come from a long evolutionary heritage, resulting in a genetic recipe and a development programme that plays out as a child experiences the world. Thus every aspect of our brain derives from brute empirical facts: from who did or didn’t get eaten by a lion, who did or didn’t produce offspring, from what happens if you stub your foot on a rock, or are rude to a playmate.
Thus the claim of Logical Positivism becomes the claim that knowledge derives from empiricism, and that any meaningful statement is one about empirical reality, and can be verified or disproved by reference to empirical reality.
The commonest criticism is then (this example taken from RationalWiki):
The most famous principle of logical positivism is that any statement that is not inherently verifiable is meaningless and can be safely ignored. Since this statement is itself inherently unverifiable, logical positivism tells us that logical positivism can be safely ignored. […]
Positivism asserts that any statement that cannot be empirically tested is meaningless. However, logical positivism is a philosophy, and cannot be empirically tested itself. By its own criterion, therefore, logical positivism is meaningless.
This objection, however, doesn’t impress the scientismist. “Logical Positivism” might indeed be a philosophy, but any truth in philosophy is just another part of science, another region of the sphere of knowledge ultimately derived from empirical reality, as logic and maths are. Thus the logic and reasoning of philosophy can indeed be validated empirically — that is the only way any logic or reasoning is validated. Thus the answer to whether Logical Positivism can be validated empirically — validated by science — is “yes”.
Scientism regards the universe as an interconnected whole. If something has no possible connection with empirical reality then it is irrelevant, it can never (ex hypothesi) have any effect on us. It can be sensibly claimed to “not exist”. If something does have some meaningful effect on us, then that effect must have empirical consequences. Thus the principle of Logical Positivism is correct: if something has no connection with empirical reality then it is certainly “meaningless” as regards our universe (if you wish, you can think of such things as being about a parallel and causally disconnected, and thus utterly irrelevant, universe).
The same criticism is often aimed at scientism, with the same claim that it is self-refuting. Scientism asserts that knowledge comes only from the scientific method (though that is interpreted broadly as including all rational enquiry based on empirical evidence). The smart-arse then asks (here’s one example): “Ok, so is that claim, that knowledge comes only from the scientific method, provable scientifically?”. The answer is: “Yes, it sure is”.
The error in seeing this question as refuting scientism comes from the erroneous categorisation of knowledge into distinct and incompatible domains (“non-overlapping magesteria”). In such a scheme scientism is a “philosophy”, so it cannot be validated from the separate domain of science.
If, though, philosophy and science are simply labels for different regions of a unified and seamless sphere of knowledge (labels akin to “chemistry” and “biology”) then there is no reason why empirical science cannot validate a philosophical claim. Indeed, as a staunch scientismist — and in the spirit of Logical Positivism — I’ll assert that any philosophy that cannot (in principle) be validated by empirical evidence is literally meaningless nonsense.
But isn’t that self-referential, science verifying itself? Yes it is. Primarily we have only our empirical experience of the universe, and we do our best to make sense of that. The scientific method is not an a priori assumption, it is a product of that enquiry, adopted because it works, because it leads to the best interpretations of the universe, and gives the most predictive power about things we don’t yet know.
The result of that enterprise is that, as best we can tell, the universe is indeed unified such that ideas of logic and evidence apply the same throughout. That is the essence of scientism, that it is not the case that one set of values, methods and logic apply in some areas of knowledge, but totally different and incompatible ones apply to other areas. Thus scientism is an empirically based deduction about how the universe is, as best we can tell.
That claim is scientific because it is refutable. If it were the case that priestly divines, working with prayer and magic runes, managed to produce better predictions of future solar eclipses than astronomers working with logic and evidence, and if the astronomers were continually baffled about why their predictions didn’t work in that domain, then the claim would be refuted.
Of course, as science has long accepted, scientific enquiry can never lead to absolute certainty. Our empirical experience of the universe will always be a small subset of reality, and we are always open to learning more that might show that a previous idea or model — developed from that partial sampling — is incomplete or simply wrong.
Subsuming philosophy into a branch of science then implies that expecting any philosophical claim to be entirely bullet-proof is fallacious. The claims of Logic Positivism about knowledge then become provisional, they are simply our best interpretation of the world around us. Since they are ultimately derived from our limited empirical experience we would not expect a logically rigorous proof of them. That statement solves many of the remaining philosophical problems with Logical Positivism.
Someone rejecting scientism might at that point assert: “Ok, so science validates itself against empirical reality, but might some other system, say theology, be able to validate itself in its own system, and thus claim equal standing with science?”.
The answer is that, if this other system were about the empirically real universe (at least partially, such that it made predictions about what would be observed), then it would compete head-on with science. One system would be found to be correct (or at least more correct) and the other would be found to be incorrect and refuted. If this alternative system were, however, not about the empirically real universe, then what is it about? It would, at best, be about some “meta reality”, or it would be about nothing at all. As regards ourselves it would be irrelevant and meaningless. That is the essential message of Logical Positivism, a message living on as scientism.