Ayer published Language, Truth & Logic in 1936 when he was only 26 years of age. He was a part of the Vienna Circle; who were notoriously known for their philosophy of logical positivism. Logical positivism is a philosophical theory that holds meaningful only those non-tautological propositions that can be analyzed by the tools of logic into elementary propositions or are empirically verifiable. It therefore rejects metaphysics, theology, and sometimes ethics as meaningless  . In Language, Truth & Logic, Ayer puts forth his own version of the verification principle. It is by this principle of verification in which these philosophers, including Ayer, assess whether or not propositions are meaningful. Furthermore it is by the principle of verification and the idea of the analytic-synthetic distinction in which the heart Ayer’s philosophy rests. Analytic propositions are propositions that are true or false in virtue of their meaning alone and synthetic propositions are propositions that are true or false in virtue of how the world is. Ayer’s logical positivist position and principle of verification is faulty and unreliable as shown by many criticisms ranging from self-refuting nature of the principle of verification to the collapse of the analytic-synthetic distinction by Quine.
To begin with, Ayer’s principle of verification goes as follows:
“a sentence is factually significant to any given person, if, and only if, he knows how to verify the proposition which it purports to express that is, if he knows what observations would lead him, under certain conditions, to accept the proposition as being true, or reject it as being false. If, on the other hand, the putative proposition is of such a character that the assumption of its truth, or falsehood, is consistent with any assumption whatsoever concerning the nature of his future experience, then, as far as he is concerned, it is, if not a tautology, a mere pseudo proposition. The sentence expressing it may be emotionally significant to him; but it is not literally significant.  “
In other words, a proposition is factually significant if it is a tautology or if it is possible to be empirically observed under conditions that would allow it to be rendered true or false. By wording it this say Ayer allows propositions such as “there is oxygen on the planet Uranus” to have factual significance because it could be possible under the right conditions to test that claim. However a claim such as “God is omniscient” does not have any factual significance and is nonsense in Ayer’s view, because it could never be verified due to God being outside of our experience. The same fate goes that of ethics and aesthetics by following Ayer’s principle of verification. By following this principle, one cannot say that “the Mona Lisa is beautiful” or that “Murder is wrong” because you cannot verify those things empirically. Also by assuming the Verification Principle it follows that scientific laws are meaningless statements, and are empty of informational content. The Verification Principle rules out all scientific laws, and therefore the whole of science. However Ayer tries to allow for such things as scientific laws by making a distinction between strong verification and weak verification. Strong verification refers to statements which are directly verifiable, that is, a statement can be shown to be correct by way of empirical observation. Weak verification refers to statements which are not directly verifiable but instead highly probable by means of empirical observation. However, the phrase ‘highly probable’ introduces a sense of subjectivity. If one can allow for such things as scientific laws as meaningful due to them being highly probable then one can allow for such things as ethics as meaningful depending on whether you think the evidence at hand is enough to deem it highly probable. An additional criticism can be made here in regards to what is meant by empirically ‘observable’. By observable do we mean with the naked eye? If so, the concept of cells must then fall to weak verification. However even if what was meant by observable allowed for such things as microscopes so that we did actually observe cells then what about the things in which we cannot observe at all such as black holes? We all believe there are such things as black holes but we cannot actually observe them but rather we observe the effects of black holes. Therefore, so to must black holes then fall to weak verification. Furthermore it is solely by seeing what follows from this principle of verification that one would desire to reject it. The greatest objection to the principle is that it is too much of a double edged sword; it cuts more good out of the world than it does bad.
American philosopher and mathematician, Hilary Putnam, puts forth the argument that making a strong and weak distinction or an observational and theoretical distinction is meaningless. Putnam argues that making such a dichotomy is a problem from the start with four objections: “something is referred to as observational if it is observable directly with our senses. An observation term cannot be applied to something unobservable. If this is the case, there are no observation terms.” So to use the term observational in regard to anything we try to verify indirectly is in itself a mistake. The term is being applied inappropriately and the result is the conclusion of there being no observational terms. “Some unobservable terms are not even theoretical and belong to neither observation terms nor theoretical terms. Some theoretical terms refer primarily to observation terms.” This is to say that there is no clear distinction always. “Reports of observation terms frequently contain theoretical terms.” Again he is collapsing the distinction here. “A scientific theory may not contain any theoretical terms.  ” In conclusion Putnam states that following the previous objections there cannot be a distinction of observational (strong) and theoretical (weak) verification.
A more moderate criticism to be made of Ayer’s principle of verification specifically comes from Karl Popper in his essay “Science as falsification”. One important fact to know about Popper is that Popper believed that scientific knowledge is provisional. That is to say, it is the best we can do at the moment. While Ayer’s principle of verification holds that meaningful statements are only those non-tautological propositions that can be analyzed by the tools of logic into elementary propositions or are empirically verifiable; Popper argues that such a method of verification is too strong a criterion for science and instead argues “that the criterion of the scientific status of a theory is its falsifiability, or refutability, or testability.  ” Instead of the method of verification being whether or not a statement is empirically observable, the method instead is whether or not it can be falsified. One should not try to confirm a theory as the logical positivists try to but one should try to falsify it. Instead one should operate by a method of deductive reasoning. Popper states, “Every “good” scientific theory is a prohibition: it forbids certain things to happen. The more a theory forbids, the better it is.  ” Following from Popper’s theory of falsifiability, the more predictions a theory makes, the better it is. The strength of Popper’s method is solely that it did not use inductive inference as the positivists method of verification did and therefore Popper’s method did not accompany the philosophical problems that come with the positivist’s method and Popper’s method, while being riddled with its own problems, did not claim statements to be meaningless but instead unscientific for the time being.
Another means of criticism features Ayer’s principle itself is its own demise. When considering the principle of verification a question comes to mind. How is it that the principle of verification is verified? The answer cannot be. The principle cannot verify itself. We could test the principle empirically but to do so we need an independent test for meaningfulness. If we don’t have an independent test for meaningfulness then we can’t test the verification principle to see if the hypothesis fits the data. Therefore, nothing verifies the principle of verification. If the principle itself cannot be verified then it is of no importance. It is nothing less than self-refuting and the whole of logical positivism collapses upon itself. A popular rebuttal to this objection is that the verification principle is more of a meta-theory rather than a theory and does not need to be verified as it is on a different level than that of theories. Any theory could be called a meta-theory just by saying so and then avoid self refuting criticism. By saying a theory is above other theories and does not need verification does not make it so. This is nothing short of a bias for the principle and not true inquiry. Carrying on from this idea that the verification principle is more of a meta-theory is the rebuttal to be made by the logical positivists that argues logical positivism is a philosophy of science and not an axiomatic system that can prove itself. However while it may not be presented as an axiomatic system that can prove itself, it is still an axiom. It is an assertion of a way to determine meaning that is not able to be proved correct. The principle of verification requires other axioms to establish the criteria of experiential proof as a prior condition, and they cannot be proven experientially or otherwise either without begging the question. Therefore, the principle of verification is meaningless.
One of the strongest criticisms to be made, if not the strongest criticism to be made, against Ayer’s logical positivism was made by W.V.O. Quine in his essay “Two Dogmas of Empiricism”. Quine’s “Two Dogmas” is often cited as one of the most important works of twenty century philosophy. Quine argued that testing a meaning were holistic and by holistic what is meant that you cannot test ideas alone by themselves. When one tests one idea you test every idea that is connected to that idea also. For example, if one tested a certain hypothesis and the data that returned was not that was to be expected that would not conclude that the hypothesis is false because something may have went wrong in testing the hypothesis. The method by which one is testing may itself be flawed and not the hypothesis. However we assume that the methods by which we are testing by are correct. These assumptions could very well be incorrect and not the hypothesis. One might argue that we can then test these assumed ideas that we have but there is no practical way that we could test all of our assumed ideas that we have while testing a hypothesis without running into an infinite regress. Quine argues that there is no scientific way to make sense of the analytic-synthetic distinction and this is the first of the two dogmas. If Quine is correct in this holism then we also test our analytic belief. However analytic beliefs are supposed to be immune from empirical testing according to Ayer! Quine argues that we have a web of beliefs in which all of our beliefs make contact with the world through experience which is to say our analytic beliefs are indeed subject to falsification. For example when testing a hypothesis such as “Grass is green”, we are not just testing that, we are testing everything that this idea is connected to. If it turns out that grass is not green we might revise one of our other hypotheses such as are our eyes working properly or are we looking at grass. Even analytic beliefs may be revised as such has happened in modern physics with quantum physics and non-Euclidean geometry. It is not impossible to revise our analytic beliefs and if we are testing these and they are not true by definition and are by experience then the analytic-synthetic distinction collapses which is fatal for logical positivists such as Ayer.
To summarize, as a logical positivist, Ayer held to a principle of verification that stated a proposition is factually significant if and only if it is a tautology or if it is possible to be empirically observed under conditions that would allow it to be rendered true or false. This principle of verification is not only an impractical philosophy to follow due to its renouncement of ethics, aesthetics and science but it is also a self-refuting one due to the principle of verification being unable to be verified and not being a tautology. It is because of these reasons that Ayer’s principle of verification and logical positivism as a whole be rejected.