We are familiar with the doctrine of behaviorism and with the phrase “behavioral science”, but we are left in the dark about what exactly behavior is. What does it mean to say that the mind is reducible to behavior or that psychology is the study of behavior? One possible answer is that behavior is motion of the body: the body moves like other physical objects and behavior is simply that motion. But this is clearly wrong: the body moves with the earth’s rotation but that isn’t behavior, and a body can be knocked over without that counting as behavior. In addition, psychology could hardly be the study of the body’s motions, since that is already covered by physics—the laws of motion apply to human and animal bodies too. If behavior has anything to do with motion, it must be a more restricted type of motion than that. The natural next thought is that behavior is purposive motion: behavior belongs with action and conduct, both of which imply purpose. The OED defines “action” as “the process of doing something to achieve an aim”, and “behave” as “act or conduct oneself in a specified way”. So action and behavior are goal-directed (“conduct” gets defined as “the manner in which a person behaves”). This accords with ordinary conceptions: we speak of a person behaving loyally, ethically, selfishly, gracefully, greedily, etc. These adverbs all connote purpose, generally motive. They are psychologically tinged, not purely “physical”. All behavior is mind involving: action as it springs from psychological traits or states or processes. This already tells us that behaviorism cannot be a variety of reductive materialism, since the very concept of behavior includes psychological factors. But it still leaves us ignorant of what behavior actually is: is it perhaps something mental expressing itself in the body’s movement? What exactly is the relationship between behavior and movement?
This is actually an obscure matter, which is perhaps why it is seldom explored. If a person generally behaves thoughtfully, is this always correlated with a certain type of bodily movement? Apparently not, since many types of movement can be deemed thoughtful. It seems reasonable to understand the relation as like what is called “multiple realization”: each episode of movement “realizes” the behavioral type behaving thoughtfully. We can think of the agent as selecting a given type of bodily movement in order to achieve his or her thoughtful aims (likewise, greedy, ethical, loyal, etc.). There is no one type of movement that corresponds to the behavioral type in question. So clearly no reduction of the latter to the former is going to be possible. But still, what then is the behavioral type? It appears to be a higher-order type involving quantification over movement types: behavior is that property of agents that involves a specific aim and which is such that some movement type realizes it. If I behave greedily, I am such that my greedy aim is realized in a specific (though variable) type of bodily movement. The behavior type is thus more abstract than the particular type of movement that realizes it, more tied to its guiding motive. It is like an action plan rather than a specific occurrence, a schema not a concrete event. A well-behaved person is someone whose actions conform to a certain abstract schema—not someone whose body moves in specific ways. If I behave loyally, I conform my movements to a certain principle or ideal or recipe—but what those movements are physically is as may be. I could behave loyally while lying paralyzed in bed so long as I fulfilled certain aims; I might not even be able to blink, but I could still make the right decisions (perhaps conveyed by a brain scanner). You can behave well without even moving your body (thus omissions count as behavior). A person might get out of jail from “good behavior” without ever moving his body in a helpful manner. But generally, there is bodily movement—though not of a fixed type. Movement is more like the means for behaving, whether well or badly; it is not identical to behavior. The very same movements might be performed by an insentient robot and no behavior would have thereby been evinced. So behavior is both mentally tinged and more abstract than movements of the body. This is why we say that someone behaved generously but not that his body moved generously. It is also why we tell someone to behave herself but not to move her body thus and so (a category mistake). This fits with an interesting feature of the dictionary definition: to behave is to “conduct oneself in a specified way”. It is not for one’s body to jerk or twitch or flow or fall—but for one to conduct oneself in a certain way. Behaving is self-conducting—the affinity with a musical conductor is apt. The conductor conducts an orchestra; the agent conducts himself using his own limbs etc. This is the idea of orchestration, control, and purposive order: behaving is conducting oneself so as to achieve a certain aim. And it has a lot to do with social setting: one typically behaves in a certain social context and this requires suiting one’s other-directed movements to one’s aims (like the conductor’s baton). Perhaps the origin of the concept of behavior lies in such social contexts; certainly ideas of comportment, grace, charm, and so on, belong there (behavior towards). This is now a far cry from the project of setting psychology on the road to materialist reduction, but it is what the ordinary concept of behavior actually involves. Behavior as commonly conceived is bound up with good behavior and bad behavior, where this is socially determined; so the concept is normative as well as psychological.  It is no accident that the word “conduct” is the preferred term in legal contexts. Action, conduct, and behavior: all three notions are linked to psychological and normative considerations—and all are ontologically more rarefied than concepts for concrete bodily motions (“his arm went up”, “his lips curled”).
Is it at all reasonable to suppose that the mind is reducible to behavior as so construed, or that psychology is the science of behavior as so construed? At least such claims are not vulnerable to anti-reductive sentiment: we can’t accuse the behaviorist in this sense of omitting the mind altogether, or of being a type of materialist. That would not be much consolation to the behaviorist motivated by a desire to reduce psychology to the physical sciences, but it might give behaviorism a new lease on life—now it can be claimed that behaviorism is non-reductive and psychologically informed. (Wittgenstein’s flirtations with behaviorism might be understood in this way.) Might the mind be explicable by means of this much richer notion of behavior–the kind of behavior we refer to by such locutions as “behave loyally” or “jolly decent behavior, old chap”? Maybe there was always an element of truth in behaviorism; it’s just that actual behaviorists misunderstood their own most central concept (not the first time that has happened). What if performance actually contains competence? What if courageous behavior logically implies the character trait of courage? What if the whole inner-outer dichotomy is misguided? Maybe the mind really is behavior once we allow behavior to extend beyond the realm of mere movement. However, attractive as that may sound, I think it is a forlorn hope. The reason is that behavior must always contrast with something non-behavioral. The deed may be the beginning, but it needs something else to co-exist with; it can’t be behavior all the way down, or from every angle. Compare biology: within biology we have the field of ethology (“the science of animal behavior”), but that is just one department of the subject. There is also biological history (evolution and ontogenesis), anatomy, physiology, biochemistry, and genetics—all separate from the study of the behavior of organisms. In particular, animal behavior must be linked to animal anatomy and physiology: behavior comes along with composition and structure, as well as fine-grained functioning. Similarly, in psychology we must consider psychological history (evolutionary psychology and developmental psychology), the structure and fine-grained functioning of the mind, its basic constituents, and its genetic roots. Psychology would simply be missing out on all this if it confined itself to human ethology. From a philosophical point of view, behavior needs an underlying substrate and structure that serves its functional ends: it cannot be free-floating and self-standing. For example, pain behavior needs to be backed by actual sensations of pain, as assent behavior needs to be backed by actual beliefs. It cannot be that pain and belief are nothing more than the behavior associated with them, even when that behavior is characterized in psychological terms. Even if behavior is thoroughly mentally imbued, it is not the same thing as the mental states that give rise to it. Maybe the mental states intrinsically involve behavior, but still they are not just behavior. It is the same with animal behavior: feeding behavior, for example, is functional and even normative (an animal ought to eat), but feelings of hunger are not the same thing as feeding behavior. Behavior is the mind projected outwards, but the mind is not behavior injected inwards. The right thing to say is that behavior and the mind are part of a complete package; specifically, behavior should not be regarded as mentally neutral. In fact, the whole dichotomy of “mental” and “physical” systematically precludes us from grasping what behavior really is. It is really quite an elusive concept, which we vainly try to squeeze into two categories corresponding to the labels “mental” and “physical”.
Much the same can be said about another concept favored by the self-styled behaviorists, viz. stimulus. Behavior is said to be elicited by a stimulus, but what is a stimulus exactly? It can’t be a physical impingement considered independently of the psychological subject—a stimulus is only a stimulus for a certain type of organism. The organism must be capable of responding to it in a certain way. The ordinary notion of a stimulus is psychologically imbued (OED, “something that promotes activity, interest, or enthusiasm”)—that is, what we find stimulating. This meaning is what makes us respond intuitively to the technical use of “stimulus”—we know quite well what it is to be stimulated by something outside us. But it turns out that this is not really what is intended (though it is traded upon); instead, the notion hovers uneasily between a purely physical meaning and the ordinary psychologically loaded meaning. In the ordinary and proper sense, a stimulus is something that evokes a psychological reaction—so it is defined by reference to the mind. It cannot, then, figure as part of a reductive definition, or as a plank in the materialist edifice. But it would also be wrong to suppose that its psychological meaning enables it to play a reductive role within the realm of the psychological: for stimuli need minds to stimulate—as behavior needs minds to express. What we have here is a tightly knit package of concepts: stimulus-mind-behavior. The concepts of stimulus and behavioral response are mentally imbued not mentally neutral; they are not physically definable. What counts as a stimulus? Answer: that which elicits a mental response.  What counts as behavior? Answer: that which contributes to achieving an aim. There is no comfort for materialism in any of this. Both concepts have a role to play in psychology, but neither of them is suitable to act as foundational concepts in a science. It is not that behaviorism would have been good materialist science if it were true; the very concept of behavior is unsuited to play the reductive role assigned to it. It is not that it is clear but inadequate; it is unclear and inadequate (except as part of ethology). The concept of behavior, as behaviorists employed it, is an ill-defined concept, a mish-mash of inconsistent elements. 
 Of course it is possible to behave in solitude, even perpetual solitude (Robinson Crusoe), but that is consistent with allowing that the concept of behavior is tied to social relations: we extend it to the solitary case, possibly imagining a potential audience. We don’t call reflex movements or involuntary tics “behavior” precisely because they are not subject to social evaluation (they are not cases of behaving badly).
 We can keep on employing the concept of behavior in scientific and vernacular contexts, but we should acknowledge that it is not the concept of something materialistically acceptable. When we speak of economic behavior, say, we are committed to a psychologically loaded concept—and not one that wears its meaning on its face. It is not at all clear what behavior is. Like other psychology concepts an aura of mystery surrounds it.