The term "sensitivity" is used here simply to indicate the cellular-level phenomenon that results from the transmembrane receptivity to electrostatic changes in the external world, not any higher-level psychology. In other words, the physiological mechanism of membrane "openness" results in cellular-level sensitivity to the biochemical environment - and this is the protophenomenon underlying higher (organism-level) psychological phenomena. By this definition, single cells have "sensitivity," whereas all living creatures (comprised of many living cells) have some form of "proto-awareness" as a consequence of the collective sensitivities made possible by many living cells. The proto-awareness of most plants, however, is the simple summation of individual cell sensitivity, with no coordination among them. Animal nervous systems, in contrast, are capable of coordinated neuronal responses to environmental stimuli and have the capacity for amplification of proto-awareness, i.e., synchronized bursts of receptivity and reactivity to their environments that might be referred to as awareness. The magnitude of awareness is thus a function of the number of neurons simultaneously activated. (An interesting implication of this hypothesis, suggested by MacLennan, is that larger neurons "feel" more strongly than smaller neurons, as a consequence of the greater ion traffic across a wider expanse of neuronal membrane. The reduced ion traffic caused by the myelin sheath in myelinated neurons would reverse this effect, but in any case a correlation between the volume of ion-flow and the strength of subjective feeling is predicted.)
The neuron learns about its biochemical situation by periodically opening its membrane and allowing the flow of ions into and out of the cell, but "feeling" and "thinking" are organism-level phenomena that the lone neuron does not experience. There are necessarily "proto-feeling" and "protothinking" phenomena at the cellular level, but only the protophenomena can be rigorously defined at the cellular level; the resultant organism-level phenomena require a different terminology. To return to the analogy with social systems, the protophenomenon of democratic control in society is voting, but the individual voter has no experience of "democratic fairness" while in the voting booth. There, he typically experiences only a yes/no choice for or against the status quo, while the "fairness" and "majority rule" characteristic of democratic decision-making arises only in the summation of the effects of many voters acting together on voting day. Although clearly related to the individual voter's activity itself, democratic justice is a society-level phenomenon that cannot be adequately described solely at the level of individual voter psychology/behavior.
At the single-cell level, the term "sensitivity" suffices to indicate the ability to react to environmental stimuli and the paramecium is a good example of such biological sensitivity. Even when the organism has a nervous system, reactivity that is undertaken solely in response to one type of external irritation is also normally described as "sensitivity." For example, not only do most insects appear to respond directly and "blindly" to direct stimulation, we commonly use the sensitivity label for animal and human responses in the context of reactions to a finite stimulus: a dog that is sensitive to a high-pitched whistle, an ex-smoker sensitive to tobacco smoke, a boy sensitive to poison ivy, a girl sensitive to remarks about her curly hair, and so on. In such contexts, we would never say "aware" or "conscious" of the stimuli; it is a direct stimulus-response pair that indicates sensitivity.
In contrast, the term "awareness" is typically used in the context of meaning both sensitive to a specific stimulus and understanding the larger context of that stimulus. The dog that is "staying" before pouncing on a morsel of food is certainly sensitive to the aroma of the food, but is also aware of the larger context from training concerning the trainer's demands. We have no hesitancy in declaring that the dog is aware of the need not to react to the stimulus of food. It is aware of the larger context - not simply sensitive to the stimulus.
But is the dog conscious? We might sometimes be tempted to give it the benefit of the doubt, but the normal test of consciousness that we apply to people is whether they can spell out the consequences of their actions. The dog is certainly aware of the trainer's presence and, on the basis of previous experience, wary of the possible smack on the nose that uncalled for advance toward the food will bring, but we remain ignorant of (and skeptical regarding) the depth of the awareness in so far as the dog cannot indicate that it has concern for the good condition of its nose if it should seize the food. A chimpanzee, on the other hand, might well indicate by pointing that it does not wish physical punishment to a certain body part - and thereby communicate to us its understanding of consequences. Even without a verbal report, we might then be willing to use the "conscious" label. If the chimp is indeed able to communicate that it is not only aware of the fact that a wrong action will lead to something unfavorable, but capable of indicating what that unfavorable outcome might actually be, then the "awareness" of danger is supplemented by a "consciousness" of lawful cause-and-effect.
In the case of training a child, we would verbally inquire to the child about what would happen if the conditions set up by the adult are not met. We would normally request a verbal delineation of the possible unfavorable consequences of taking another cookie out of the cookie jar before we would be satisfied that the child is conscious of the full meaning of the situation. The child is not simply "aware" and "wary" of the fearsome power of the adult: he is certainly aware of some danger if behavior is incorrect, but, in addition to animal awareness, if the child can verbally explain consequences, we are then normally willing to say that he is conscious of his own actions.
By these definitions, sensitivity refers to the receptivity of individual cells. Without a coordinating mechanism, the quantitative increase in sensitivity due to an increase in the number of cells does not lead to higher psychological properties; the cell-conglomerate may be "very sensitive," but it cannot be aware unless its many-cells act together - are synchronized in relation to the coordi nated activity of other neurons. By its very nature, the focus of sensitivity needs to be contrasted with a larger context. In other words, "foreground" objects are in the foreground because they stand out from a background; it is the presence of the background that makes the foreground more than a lone stimulus. In this view, the reptilian brain that can respond with the coordinated activity of a large number of motor neurons arguably lacks "awareness" in so far as the motor response cannot be suppressed. But the mammal capable of suppressing the motor response arguably has the neural mechanisms necessary for awareness - an ability to see the forest while responding to the trees. Finally, human consciousness denotes a capacity not only to suppress motor responses - and thus take the delaying action that can allow one to plan and make decisions, but also to simulate the decision mechanisms before or after the event. In order for a psychological process to be simulated, however, there must be mechanisms available for re-presenting the sensory and motor events. In other words, language or some other mechanism for symbolic representation is a prerequisite to consciousness and is thus a mental capacity unique to human beings.
It is a curious and wonderful fact that the French have one word, "conscience," to mean both consciousness and conscience. If the meaning of consciousness, as distinct from awareness and sensitivity, lies in the ability to understand consequences, then the psychological link between consciousness and conscience is clear. In English, conscience means to have a retrospective ability to trace back consequences once a decision has already been made (and action taken). A "guilty conscience" therefore implies a sense that the current situation can be traced back to a specific decision made by the individual. It was a conscious, remembered decision to set out in one direction, not the other, that led to the current state. To have consciousness means to have the potential for conscience - and that is why we are more forgiving of naughty young children and animals - who typically show no remorse suggestive of conscience - than we are of equally remorseless criminals that demonstrate clever forethought and scheming purposeful decisions - i.e., consciousness afore, but no retrospective conscience. A certain class of criminal has forward-thinking consciousness in abundance, but does not demonstrate even rudimentary conscience, while young children and animals (and arguably a certain class of adult criminal) simply lack the purposeful planning - and understanding of cause-and-effect -of consciousness, and necessarily also lack conscience - a sense of remorse. Without consciousness, there can be no conscience.
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