He joined Schlick's Vienna Circle and suggested a better term than "logical positivism" would be "logical empiricism," with its emphasis on scientific evidence. Feigl visited Harvard in 1930, where he met Willard van Orman Quine, author of the famous essay "Two Dogmas of Empiricism."

Feigl was at the University of Minnesota for many years, where he founded the Minnesota Center for Philosophy of Science with Wilfrid Sellars and Paul Meehl.

Like most analytic philosophers, Feigl thought philosophical problems were reducible to problems of language. Analytic language philosophy was often defined as "therapeutic" or "hygienic." Feigl's philosopher colleagues resented his defining "philosophy as the disease of which it should be the cure," (though this was Ludwig Wittgenstein's "therapy" idea).

In 1949, Feigl and Sellars edited perhaps the most influential single volume defining logical empiricism and the new analytic language philosophy, their celebrated Readings in Philosophical Analysis." The introductory chapter, "Logical Empiricism," was written by Feigl.

The most important, the most widely debated, and, unfortunately, the most frequently misunderstood regulative principle used by Logical Empiricism is the criterion of factual meaningfulness. The purpose of this criterion is to delimit the type of expression which has possible reference to fact from the other types which do not have this kind of significance: the emotive, the logico-mathematical, the purely formal, and—if there should be such—the completely non-significant. If it is the ostensive steps that connect a purely formal array of signs (e. g., words) with something outside of language, no sign or combination of signs can have factual meaning without this reference to experience. Furthermore, if a sentence is considered true when it corresponds to an existing state of affairs, a sentence is factually-meaningful only if we are in principle capable of recognizing such states of affairs as would either validate or invalidate the sentence. If we cannot possibly conceive of what would have to be the case in order to confirm or disconfirm an assertion we would not be able to distinguish between its truth and its falsity. In that case we would simply not know what we are talking about. C. S. Peirce's pragmatic maxim, formulated in his epoch-making essay, "How to Make Our Ideas Clear," has essentially the same import.

Feigl (and Peirce) may have been the origin of Gregory Bateson's famous "difference that makes a difference."

We may paraphrase it crudely: A difference that is to be a difference (i. e., more than merely a verbal or an emotive one) must make a difference. Or, a little more precisely: If and only if assertion and denial of a sentence imply a difference capable of observational (experiential, operational, or experimental) test, does the sentence have factual meaning. Another useful formulation is Ayer's: "It is the mark of a genuine factual proposition . . . that some experiential propositions can be deduced from it in conjunction with certain other premises without being deducible from these other premises alone." This is simply empiricism brought up to date.

("Logical Empiricism,"Readings in Philosophical Analysis, p.9)

In his address to the American Psychological Association in 1958 (titled "Philosophical Embarassments of Psychology"), Feigl offered as an illustration of philosophical disease the "hoary puzzle" of free will vs. scientific determinism that had lately exercised even a few prominent psychologists.

The perplexity of this ancient issue consists in the apparent logical incompatibility between two beliefs, each of which appears plausible on its own grounds: The assumption of free choice seems borne out by the testimony of introspection; also it seems indispensable as a presupposition for moral responsibility. On the other hand, a great deal of biological and psychological evidence points in the direction of a fairly strict determinism in regard to human behavior. "Nature" and "nurture" (i.e., hereditary constitution and all environmental influences up to the moment of choice or action) are assumed to go a long way toward determining our decisions as well as our conduct. But, so it seems to many thinkers, if we are to be free, we cannot be enmeshed in a strict network of causal relations. Hence, the relief and jubilation in many quarters when the "good tidings" of indeterminacy in basic physics were proclaimed.

But a little critical reflection shows readily that this sort of "absolute chance," far from constituting free choice, would be experienced as a queer kind of compulsion, and thus not serve at all as a basis for moral responsibility (i.e., praisability or blamability). Only if, to a significant extent, we are the choosers of our choices, and the doers of our deeds, can we be held accountable. The entire bafflement is due to a confusion which can be easily dispelled.

This is the
standard argument
against free will.
If we are determined, we are not free. If we are random, then we are not responsible.

We must not confuse freedom with indeterminacy (i.e., the absence of causality), and we must not confuse causal determination with compulsion, coercion, or constraint. As already Spinoza essentially saw it, we are free to the degree that our choices and our conduct are determined by our character and personality. The fact that our personality and character in turn may have been completely determined by antecedent conditions does not militate against regarding our actions as a consequence of what we are at the moment of action. Apples are produced by apple trees, even if apple trees themselves are the products of seeds, soil, air, rain, and sunshine. To be unfree means no more than to be under some sort of constraint. To be free means that the chooser or agent is an essential link in the chain of causal events and that no extraneous compulsion—be it physical, biological, or psychological—forces him to act in a direction incompatible with his basic desires or intentions.

It has been asked how one can be held responsible for actions springing from a personality whose structure might have been determined by the initial and boundary conditions long before the infant was born. The answer to this question can be found by a logical analysis of the concept of responsibility. We are responsible to the extent that our behavior is responsive to the usual sanctions of society. Rewards and punishments, encouragements and discouragements of any sort can be successfully applied only under the conditions of at least a high degree of causal determinism. The nonresponsibility of the psychotic as well as the responsibility of the normal person can be understood only within the causal scheme of events. (The borderline cases of neurotic compulsion may be intriguing, but have to be dealt with pragmatically and in the light of the nature of the special situation on hand.) Empiricist philosophers have long ago explicated causal necessity in terms of lawfulness and predictability.

R.E. Hobart's determination is often confused with determinism, as Feigl does here.

Free will, properly understood, "presupposes causal determination (in this sense) and is inconceivable without it" (Hobart, 1934). If human acts to any extent, and through some amplification processes, were a result of basic quantum indeterminacies (i.e., on the microlevel of atomic interactions), then to that extent we could not be held responsible for our conduct.

This is G. E. Moore's claim that "I could have done otherwise" simply means "I could have done otherwise, if the past had been slightly different and I had chosen to do otherwise."

The phrase "I could have acted differently," as used in common parlance, does not imply indeterminacy of this or any other sort. It merely means "If I had been wiser or if I had had different attitudes, I would have acted differently." Under the influence of positive and negative reinforcements (some of which may well be internal to our personalities) we learn how to adjust ourselves for future exigencies and contingencies. The sentiments of regret or remorse can be instrumental in this respect only if they do not hopelessly and exclusively fix the individual's attention upon his past deeds, but rather if they are prospective and thus mobilize the resolution to act differently on future similar occasions. Docility, the capacity of modifying both one's beliefs and one's attitudes under the influence of cumulative experience, is of the very essence of freedom.

I hope this brief sketch will have shown that an examination of the meaning of the terms we use can go a long way toward the clarification of deeply puzzling issues. Proper attention to the two questions "What do you mean?" and "How do you know?" quite generally constitutes the most effective strategy in modern philosophical analysis. (It should be noted, however, that this is nothing really new in philosophy, Ever since Socrates conversed with the bright young men of Athens, inquiries into meaning and validity have been an important part of the philosophical endeavor; but this search for clarity has often been overshadowed, if not suppressed, by the extravagancies of speculation concerning absolute truth, reality, or values.)

(American Psychologist, Volume 14, issue 3 (March 1959), pp.116-117)

Feigl and Meehl wrote a paper in 1974 entitled "The Determinism-Freedom and Mind-Body Problems,"for Paul Schilpp's festschrift volume on Karl Popper. Popper had for many years been a Dualist and Interactionist.

Following Popper's usage, they called these problems Compton's problem and Descartes' problem.

They argued that determinism could not be rejected simply because of the practical difficulty carrying out predictions of the future that in principle could be done by a Laplace super-intelligence. They called the accomplishments of such a prediction the "World Formula."

They frame a pseudo-logical argument that the World Formula requires 1) determinism and predictability, specifically 2) measurability and 3) calculability. But because we cannot accomplish a World Formula, they say, we do not invalidate determinism.

Now, as is agreed on all hands, the idea of the World Formula is to be understood as a logical conjunction of three propositions: (1) the doctrine of the deterministic form of all basic natural laws, (2) the precise, complete (and simultaneous) ascertainability of all initial and boundary conditions, (3) the mathematical feasibility of the hopelessly complex computations necessary for precise and complete predictions (or retrodictions). Now, as no one knows better than Popper (though this is really a matter of the most elementary propositional logic), if a conjunction of several independent propositions entails a false or absurd conclusion, not every one of the conjuncts is (necessarily) false... There are excellent reasons for regarding propositions (2) and (3) as false at any rate, thus leaving the hypothesis of determinism at least open for further consideration.

("The Determinism-Freedom and Mind-Body Problems.", reprinted in Paul E. Meehl, Selected Philosophical and Methodological Papers, 1991, p. 98-99)