This principle is called "positivism". But I prefer the term "logical positivism".
Positivism is a basic principle of thought--- it distinguishes questions which are meaningful and meaningless. It is not meaningful to ask "How does Argentinian property law taste?", it is not meaningful to ask "How many angels can dance on the head of a pin?", and it is not meaningful to ask "Wny is there something rather than nothing". These questions have the property that, whatever the answer, there is no consequence to our perception of the world, or to anything that we can measure. Such questions do not need an answer, because they are just words put together without meaning.
Philosophers do not accept positivism, because it moots many of their favorite questions. For example "how can we have free will in a deterministic universe?". This question, although superficially so clear, is positivistically meaningless, because there is no measurement which can be made upon a creature's decision making power which can in any way determine whether the underlying laws are deterministic or not. This question is moot, because if you define "free will" properly, it cannot possibly have anything to do with determinism. You can't figure out that quantum mechanics is not deterministic by introspection, because your introspection has nothing to do with the determinism of the underlying laws.
- If the universe exists, doesn't it require a prime mover?
- Is ethics/mathematics defined independently of human beings, or do we produce it?
- If God is good, how come there is so much suffering in the world?
Basically, half the philosophy curriculum is brain-dead nonsense, and the rest requires massive restructuring to eliminate redundancies. Religion, when formulated positivistically, is not nonsense, and it is possible to formulate religion, including monotheistic religion, in positivist terms, only throwing away the metaphysics and keeping the meat of it.
Logical positivism is an early twentieth century tradition that combined the notion of scientific positivism with the new concept of a formal language, then being developed in Logic by Boole, Frege, Russell, Hilbert, and others. The logical positivists wished to get rid of the other annoyance of philosophy, the fact that we have to deal with imprecise natural language. The goal was to replace natural language with a formal language, in which all the terms of discourse are precisely defined. In this way, you would remove another huge branch of philosphy, namely that philosophy whose goal is to make sense of the various ambiguities in the writings of other philosphers.
The formal languages at the time were barely adequate for formalizing mathematics. It was a bold leap to assume that formal languages could encompass a large enough domain of discourse to approximate natural language. This leap was associated with a bunch of people who I am not going to name, because I haven't read any of them, because philosophy is so trivial compared to any real intellectual work, that you can reproduce any of the results of the non-mathematical philosophers for yourself by thinking for ten minutes.
Logical positivism moots a whole bunch of other philosophy, because it suggests that the right way to think of terms in a language is in terms of a reduction to a formal language, much like in mathematics. If you take the positivists seriously, most of the field of philosophy is pointless and stupid.
At first, philosophers celebrated the revolutionary ideas, and positivism was the ascendent philosophy until the 1950s. But with the emergence of horrific totalitarian states with scientific materialism as their religion, scientific materialism lost ground, and by the 1970s, it was killed off in the west.
The positive influence of positivism
People think positivism equals quantum mechanics, because quantum mechanics was formulated strictly in positivist terms, using "observables". But it wasn't only in quantum mechanics that positivism was important:
- Fields: the notion of fields, which is so abstract, acquires meaning through positivism. How can you tell if a field is there? Put a charge there and see it move! This positivist formulation was important in making clear that a concept that seems so immaterial to many people at first glance is in fact real.
- Luminiferous ether: the ether lost its material characteristics one by one, and it was an act of positivism by Einstein to reject the ether completely, because it had become unobservable.
- Equivalence principle: In order to get from the fact that you can't observe gravity in a free-falling frame to the principle that gravity is a geometric force, Einstein made an act of positivism. If the effects of acceleration are indistinguishable from gravity, then gravity and acceleration must be the same thing in essence. This is a very predictive statement.
- String theory: String theory emerged from a positivistic question--- how can you make a measurement in a space-time that is not well defined at short distances? The answer was to speak about s-matrix states, and their scattering. In physics, the resulting S-matrix theory led to string theory, which is the only candidate for a theory of everything.
- Black hole interiors/holography: Within string theory, the rejection of the simultaneous existence of the interior and exterior (since each are observed by different observers) led to the fruitful principles of black-hole complementarity and holography as developed by Susskind.
There are also several cases where positivism was misapplied overzealously, and I think this clarified things:
- Quantum field theory is meaningless--- because it is difficult to imagine measuring a quantum field. In fact, the original paper of Bohr-Rosenfeld analyzed measurements of the quantum electric and magnetic field, and decided that it made sense to quantize them. the positivist's complaint was about ultra-short distances, and for those distances you need to deal with gravity anyway.
- Quarks don't exist--- it is impossible to isolate quarks. This is no better an argument than Mach's argument that atoms don't exist because he can't see them. If there are observable phenomena which are best explained using quarks, then quarks exist to the extent that they are included in the explanation. The real battle here was over quantum field theory, and the postivists were the folks doing the s-matrix theory.
Outside of physics, positivism is identified with this ridiculous claim:
- Interior experience doesn't exist--- since all we can measure are inputs and behavior, we should model organisms as a black box without internal experience. This idea, due to Skinner, is pretty idiotic.
In my view, positivism is an engrained part of science, and in my view, it has never steered us wrong, even in cases like S-matrix-theory/string-theory when almost everybody thought it did. With the presence of computers, and computer languages, formal languages are no longer abstract and remote. People program computers all the time. Scientists know when what they say has meaning when they can program a computer to do their model. So I think it is fair to say logical positivism is the only correct functioning philosophy.
All the logical positivists are now dead, and their work is basically ignored within their own field. If you ask philosophers why they ignore logical positivism, they often say: "Positivism contradicts itself. It says all truths need to be experimentally verifiable, but positivism is not experimentally verifiable!" To call this stupid, self-serving and intellectually vapid is too kind.