General Reactivity Theory
Consider the simple reflex: the blink reflex or the patellar reflex, for example. There is a stimulus and a response: the stimulus is an impinging physical event and the response is a movement of the body. The stimulus elicits the response without any psychological intermediary; the reflex arc exists outside of consciousness and will, automatically, inexorably. It is a case of straight physical causation. But it is not quite as simple as it may appear, since more structure must be postulated than mere stimulus and response (plus linking pathways). First, the stimulus must be detected and recognized for what it is, if not by the person then at least by the person’s nervous system—there needs to be a stimulus receptor. Second, the response is not produced purely by the physical properties of the stimulus but by a suitable response generator—a mechanism for triggering an appropriate movement of the body. It would be no use if a tap on the knee caused the eyelid to close or if an incoming missile heading towards the eye caused the knee to shoot up! The generator must deliver a response that corresponds to the stimulus; nothing about the physics of the stimulus alone determines how the body will react. So the twofold stimulus-response structure is really a fourfold stimulus-receptor-generator-response structure. And note that the extra two ingredients are more internal to the organism than the stimulus and response as such, and they are more information involving. Furthermore, it would be wrong to characterize the stimulus-response nexus as merely a cause-effect nexus, as if there was nothing special about stimuli eliciting responses compared to physical causes bringing about physical effects. Lightning hitting a tree and scorching it is not an instance of stimulus and response, as falling to earth is not a response to the stimulus of gravity (the motion of the planets is not a response to the stimuli supplied by the sun). The stimulus-response relation is a special type of causal relation, if it is a causal relation at all. The obvious point is that it is purposive—a matter of design, adaptive and teleological. The response reflects the needs of the organism and contributes to its survival. We only call an event a stimulus if it affects organisms in certain ways not because of its physical parameters. And what counts as a stimulus for one type of organism may not count as a stimulus for another type, depending upon its receptivity and responsiveness (e.g. sounds that are too high for humans to hear or light that lies outside the humanly visible spectrum). The concepts of stimulus and response are proprietary to living systems and involve teleological notions. A stimulus is not just any old cause and a response is not just any old effect. The OED puts it nicely: a stimulus is defined as “a thing that evokes a specific functional reaction in an organ or tissue” (deriving from the Latin “goad, spur, incentive”). In short, it is a biological notion. The word “respond” is also defined by the OED in loaded terms: “to say or do something as a reply or as a reaction”. Correspondingly, a stimulus is said to elicit a response not merely to bring it about, as a response is a reaction to a stimulus not merely a consequence of it. These are all biologically loaded notions by no means equivalent to physical concepts. Living organisms are the proper subjects of these notions, and they purport to describe the specific nature of such entities. Even the simplest reflex is conceptually rich in the way outlined.
The question I am concerned with is whether this network of notions has a wider application in describing the operations of mind. To put it with maximum bluntness: is the mind a stimulus-response system? I shall suggest that it is, so the simple reflex can act as a model for the general character of the mind. I intend this to sound outrageous, given the uses to which the notions of stimulus and response have been put, but on reflection we may have thrown the S-R baby out with the behaviorist bathwater. This may still be a useful and accurate way to talk, even though it has been multiply abused. The first point to note is that it has nothing essentially to do with any attraction to materialism or behaviorism: stimuli and responses may be psychological in nature, irreducibly so. Nor need they be observable or measurable or public or experimentally usable. For instance, we may reasonably say that pain is a response to a harmful stimulus, even if the pain is unobservable and non-physical–even if it is entirely immaterial. The point is that the sensation is automatically elicited by the stimulus—the two things stand in the S-R relationship. Likewise, a sensation can act as a stimulus eliciting a response, as when a pain elicits a cry or an itch elicits scratching. In fact, it is entirely appropriate to describe perception in general as a reflexive stimulus-response system: the impinging stimulus, say irradiation of the retina, elicits a sensory response, say seeing a red object. This is a “functional reaction” to an incoming stimulus—and the physical impingement acts as a stimulus for the organism in question. The perceptual response is an adaptive reaction to the organism’s environment, entirely analogous to the blink reflex or the perspiration reflex or the flinching reflex or the disgust reflex or the salivation reflex. Seeing an object is a stimulus-response linkage. And let it be noted that the extra layers of receptor and generator are present here too: the senses need receptors to register the stimulus and a mechanism to generate the percept that results. There is nothing behaviorist about this in the classic sense. It is unapologetically mentalist.
The interesting question is how far the S-R model can be extended, and here some controversy can be expected. Let’s consider belief, emotion, and intentional action. Belief can be viewed as a response to the stimulus afforded by perception: you see a red object and respond by forming the belief that there is a red object there. We need not suppose that forming a belief is an action—it is not—but we can suppose that it is a reaction to a percept; not all reactions are volitional. The cognitive system is set up in such a way that beliefs are triggered by perceptions: beliefs are “functional reactions” to the stimuli afforded by the perceptual apparatus. The seeing elicits the believing. In the case of beliefs that arise by inference we can say that the conclusion belief is a response to the premise beliefs: beliefs can function as stimuli that evoke other beliefs as response. Again, this is adaptive and functional—as is very evident for animals solving problems by reasoning. The premise beliefs don’t just cause the conclusion belief; they act as stimuli that elicit that belief—that is, they are part of a functional biological system. In some cases the inference pattern may be instinctual, in others learned, but it is an S-R arrangement in either case. In the case of emotions, the response is triggered by an external stimulus, say a threat; and the response may be rapid, automatic, and unavoidable (again think of animals). The emotion is an evoked response, also functional. Fight or flight responses are mediated by emotion, and the emotion is as much a response as the behavior that goes with it. We ask, “What was your reaction?” when hearing about some untoward experience of a friend, and expect to be told what emotions were evoked. This is just stimulus and response, though of a more complex and mediated nature than simpler cases. In the case intentional action we can introduce need and desire as stimuli: the organism is prodded to act (goaded or spurred) by its internal appetitive states, say hunger or amorous desire. The desires act as stimuli to the volitional (motor) system and they serve to elicit appropriate actions. These stimuli can vary in intensity as perceptual stimuli can; the response evoked is thereby modified and enhanced. Logically, the case is just like other S-R linkages—biologically functional causal patterns. And again it will be necessary to postulate receptors and generators as well as the stimuli and responses themselves—all the machinery of response elicitation.
There is thus a recurring pattern in the functional architecture of the mind: stimulus-response relations elaborately organized, varying from simple to complex. There are chains of such patterns, as a response becomes a stimulus to a further response, which in turn becomes a stimulus. We must purge ourselves of old associations of these notions deriving from an antiquated behaviorism. No doubt the early behaviorists were influenced by nineteenth-century biology, in which the idea of biological responsiveness played an important role—as in early studies of the nerve impulse. Neurons were discovered to work by means of stimulus and response, as one neuron abuts another and evokes action potentials in it. Tissue in general was described as “irritable”—reactive, alive, lively. The behaviorists then took this useful way of thinking and converted it into a positivistic picture of public bodily events. But the conceptual apparatus itself is quite independent of this move, merely recording the biological fact of one thing eliciting another in a functional manner. The whole organism is composed of reactive organs that respond in certain ways when stimulated in certain ways, the brain being no exception. It is then a short step to regarding the mind, itself a biological organ, as likewise an array of S-R linkages. This enables us to take the mind down from pre-Darwinian obscurity and religious obscurantism (the soul etc.) and locate it within the biological organism. We thus obtain a healthy biologism about the mind not a doctrinaire behaviorism (unless we choose to liberate behaviorism from its materialist and positivist dogmas by opting for internalbehaviorism). It is true that the notions of stimulus and response must be extended considerably from the case of the simple reflex—in particular, in relation to the automatic character of such reflexes—but there is no logical bar to accepting that some S-R connections may be less fixed and invariable than others. There can be probabilistic response elicitation, even resistance to some types of potential stimuli (e.g. strong but unwelcome desires). We can allow that theory formation in the sciences counts as an advanced form of response elicitation by the stimuli offered by the evidence, odd as it may sound to talk this way. The S-R schema does not stop at the higher cognitive functions when suitably generalized. In addition, it should not be supposed that the mind admits of no other useful mode of description: we can certainly acknowledge that there are mental competences, mental faculties, and mental qualities. It is just that mental operations have a stimulus-response structure: mental transitions are always governed by S-R logic. Even the humble patellar reflex needs its underlying machinery—competence, if you like—and linguistic stimulus-and-response undeniably relies on an underlying structural competence. The same is true of organs of the body: each needs its specific architecture and cellular substrate in order to permit the stimulus-response connections in which it engages. But when we describe an organism as a situated living thing, acting and reacting in the world, transitioning from one state to another, we need the conceptual apparatus of stimulus and response. All I have done here is suggest applying it more widely than is customary. For it provides a nice unifying framework for thinking about the mind, shorn of all connection with behaviorism, conditioning learning theory, anti-nativist empiricism, and anti-cognitive bias. Cognitive science turns out to be S-R psychology after all, when properly understood.
 The stimulus-response concept is not the same as the input-output concept. The latter concept is more general, applying to non-living systems as well as living ones, and it lacks the teleological connotations of the former concept. It derives more from computer technology than traditional biology.
 In psychology it is customary to distinguish the proximal and the distal stimulus, e.g. the light proximally impinging on the retina and the distant object sending out that light. The same kind of distinction can be applied to the full range of S-R relations: the proximal stimulus to a belief might be a conscious perceptual state while the distal stimulus is a pattern of light on the retina. Also we can define the same kind of distinction for responses: the proximal response for a percept might be a belief while the distal response is an utterance expressing that belief. The same distinction can be applied to desires and emotions, where there can be closer and more remote stimuli and responses.
 I don’t mean to suggest that all mystery is removed thereby, only that the mind is set in its proper place as a natural attribute of organisms. We get a conceptual continuity between the various aspects of organisms.
 This enables us to inject a welcome dose of biology into cognitive science, which tends to view the mind as inherently divorced from the processes of life, like a computing machine. It is not as if theorists have adopted a computer model for the activities of the body. Cognitive science has in effect erected a new form of dualism, which the S-R schema helps us transcend.