Willard Quine described, in his article "Ontological Relativity" (Journal of Philosophy 65(7):185-212), his doctrine of the indeterminability of translation. Roughly, this says that words are meaningful (a collection of words emitted by an agent can help predict that agent's actions), but don't have meanings (any word taken by itself corresponds to nothing at all; there is no correspondence between the word "rabbit" and the Leporidae).
In Quine's words,
Seen according to the museum myth, the words and sentences of a language have their determinate meanings. To discover the meanings of the native's words we may have to observe his behavior, but still the meanings of the words are supposed to be determinate in the native's mind, his mental museum, even in cases where behavioral criteria are powerless to discover them for us. When on the other hand we recognize with Dewey that "meaning. . . is primarily a property of behavior," we recognize that there are no meanings, nor likenesses nor distinctions of meaning, beyond what are implicit in people's dispositions to overt behavior. For naturalism the question whether two expressions are alike or unlike in meaning has no determinate answer, known or unknown, except insofar as the answer is settled in principle by people's speech dispositions, known or unknown.
Quine got my hackles up by using the word "naturalism" when he meant "behaviorism", implicitly claiming that naturalistic science was synonymous (or would be, if he believed in synonyms) with behaviorism. But I'll try to remain impartial. (Quine's timing was curious; Chomsky had demolished behaviorist linguistics in 1959, nine years before Quine's article.)
Quine's basic idea is insightful. To phrase it in non-behaviorist terms: If all words are defined in terms of other words, how does meaning get into that web of words? Can we unambiguously determine the correct mapping between words and meanings?
Quine's response was to deny that that is an empirical question. He said you should not even talk about meaning; you can only observe behavior. You must remain agnostic about anything inside the head.
But it is an empirical question. With math, plus with some reasonable assumptions, you can prove that you can unambiguously determine the correct mapping even from the outside. In a world where you can tell someone to think of a square, and then use functional magnetic resonance imaging and find a pattern of neurons lit up in a square on his visual cortex, it is difficult to agree with Quine that the word "square" has no meaning.
You may protest that I'm thinking there is a homunculus inside the mind looking at that square. After all, Quine already knew that the image of a square would be imprinted in some way on the retina of a person looking at a square. But I am not assuming there is a homunculus inside the brain. I am just observing a re-presentation inside the brain. We can continue the behaviorist philosophy of saying that words are ultimately defined by behavior. But there is no particular reason to stop our analyses when we hit the skull. Behaviors outside the skull are systematically reflected in physical changes inside the skull, and we can investigate them and reason about them.
The more I tried to figure out what Quine meant - sorry, Quine - the more it puzzled me. I'm with him as far as asking whether meanings are ambiguous. But Quine doesn't just say meaning is ambiguous. He says "there are no meanings... beyond what are implicit in... behavior". The more I read, the more it seemed Quine was insisting, not that meaning was ambiguous, but that mental states do not exist - or that they are taboo. And this taboo centered on the skull.
Behaviorists claimed that they forbade reasoning about states that were not observable by behaviors. But what they forbade, in practice, was reasoning about states that were inside the skull, whether or not they were observable. They did not say that we did not yet have the technology to study the effects of stimuli on the brain (we did). They said there were no mental objects inside the brain.
This got me wondering what a behaviorist means by a mental object. To me, meaning is something that gets more and more definite the more you know about how brain states correspond to environmental states. "Ambiguity" is not a binary predicate; it's the number of possible solutions, or the variance of a probability distribution. But to them, meaning is something that is not approximated or approached, but that you either do or do not apprehend. And, no matter how far back you trace the causal chain, you will never arrive at it. It's pure, possibly atomic... transcendental.
They mean the soul.
Most religious are closely associated with the idea of the immaterial, and more specifically, an immaterial soul that contains a person's essence. Modern re-interpretations of this idea, from Descartes to John Searle, seek to locate the soul within the brain. The soul is a way to protect the human mind, and free will, from reductionism.
Historically, I was taught, behaviorism arose as a response to intuitionism, which took people's thoughts about their thoughts as evidence. It was a methodology designed to rule out errors caused by intuitionism.
But behaviorists went much farther than that. They didn't try to delimit the situations where intuitions were useful, or where one could talk about mental structure or content. They insisted that all talk of mental representations or computations were taboo. They were at odds even with the science that already existed at the time. It was already known, though not with much precision, that damage to particular parts of the brain caused particular mental malfunctions; as did brain diseases with different histopathologies.
So perhaps this original, good methodological idea fell victim to a nearby, stronger attractor: Behaviorism became the flip side of the concept of the soul. You can try to protect the mind from reductionism by encapsulating it in a soul. Or you can protect the mind from reductionism by denying that it exists. (You can talk about my behaviors all you want; just don't look inside my skull.)
I wouldn't say that everyone who called themselves a behaviorist had religious motivations. But perhaps there was a dangerous synergy between his rationalist doctrine, and a parasitic religious one.
But B.F. Skinner, not the earliest but the most well-known evangelist of behaviorism, does not strike me as a religious man. And Skinner denied the existence of free will, which is the opposite of what I expect people trying to protect the mind from scrutiny to say.
So here is an alternative theory: Behaviorists were religious cynics.
A cynic promulgates views that sound sociopathic. Yet a cynic is not a sociopath. Sociopaths are not bitter about failing to find cosmic meaning in the world around them. Cynics are idealists who want too much from their ideals - realists longing for the transcendantal.
Seen through this analogy, behaviorism is a defense of the soul by people who don't really believe it exists; a denial of free will that doesn't solve the tough philosophical problem, but tells people to look away: "Move along, nothing to see here". Like religion, it justifies not thinking about threatening philosophical questions.