In a large proportion of other cases, how ever, our interpretation of our tactile sensa tions, especially of all those which relate to the configuration, density, &c. of external ob jects, is based on experience ; and those who watch the eagerness with which the infant grasps and examines by its touch every attrac tive object within its reach, are at no loss to perceive how the experience thus early inter woven (as it were) with the mind, in combin ation with that derived through the visual sense, comes to supply the place of the con genital intuitions of the lower animals, and to cause the tactile and visual perceptions to be henceforth so indelibly associated, that each is continually suggesting the other. Thus, the notion of projection, which we derive through the sight, comes to be associated with that of solidity, which we receive through the touch ; and the visual notion of polish is so closely connected with the tactile notion of smoothness, that the one almost necessarily suggests the other. There is abundant evi dence, however, that there is no or intuitive connection between the ideas which we derive through these two senses respec tively; but that this connection is acquired by the consentaneous exercise of them. Thus, from observations made upon persons born blind, when visual power has been first ob tained, it is certain that the notions of form previously acquired by the touch do not aid in the visual discrimination or recognition of objects : so that, for example, if any such person had previously learned to distinguish a sphere, a cube, and a pyramid, by the touch, he would not be able to say which was which by looking at them, until he had learned by experience to associate the two classes of per ceptions : and, conversely, we cannot but be lieve that the same result would occur, if a person whose notions of the external world were derived from the sight alone, were sud-' denly and for the first time to become en-' dowed with the sense of touch.
It is, in fact, no less clear in regard to the sense of touch than it is in regard to vision,' that it is not the sentient organ (as we are accustomed to term it), but the mind, which really perceives ; and that all the notions which we derive through this sense with re spect to external objects, whether they be of the most general kind or of a more particular nature, are altogether distinct from the sensa tions themselves. It has been well remarked by Professor Alison, that "one decisive proof of this being the true representation of this part of our mental constitution is obtained by attending to the idea of extension or space,' which is undoubtedly formed during the exer cise of the sense of touch, but which is no sooner formed than it swells in the human mind to infinity,' to which, certainly, no human sensation can bear any resemblance."# So, again, the elementary .notion of an ex ternal universe as something distinct from the individual self, is altogether distinct from the sensations which excites it. All that the mind is conscious of, is a change in the condi tion of the corporeal organism ; and the re ference of the source of this change to some external agency is a mental process in which the action of the purely sensorial apparatus has no concern.
It has been thought by some that the notion of an external world depends more upon the sensations received through the touch, than upon those of any other kind. But there does not seem to the author to he any reason for considering that simply tactile impressions are inure necessarily or intuitively recognised as proceeding from an external source than are the visual, olfactive, auditory, or gustative. But, as already shown, it is from the muscular sense that we derive the idea of force, involv ing resistance to our own voluntary efforts ; and it would seem to the writer to be on this notion that our belief in the existence of an universe external to ourselves most securely rests.
The active co-operation of the mind is re quired, not only for the formation of the notions so immediately springing from sensa tions as to be often confounded with them, but also for the reception of the sensory impres sions themselves. Until, in fact, the mind has been affected by these impressions, no sensation can be said to exist ; and that of which the mind takes cognisance is not the external object but the impression produced by it, and not the direct or immediate impres sion produced by it upon the organ which first receives it, but the change in the senso rium consequent upon this. (See SENSA TION.) That this is the true account of the process is now universally admitted both by the psychologist and the physiologist ; and it is placed beyond all reasonable question by the occurrence of those subjective sensations, which, until their indications are corrected by experience, may suggest the idea of an external source, with such vividness and de finiteness, that the objective unreality can scarcely be credited. In some instances the excitement of these subjective sensations ap pears due to the occurrence of a change in the part in which they are felt, which simu lates that which would be produced by an external impression ; as in the case of the sen sation of extreme heat, which is often expe rienced in inflammation to a degree far beyond that which the actual exaltation of tempera ture would account for ; and the pain, of va rious kinds, often resembling that inflicted by external injuries, which is the result of morbid changes in the part to which it is referred. But in other cases they are clearly referable to changes taking place in the course of the nerve-trunk to the sensorium, which simulate those which would naturally occur in it when it is the conductor of an external impression ; of this several examples have already been given. Or, again, they may be due to a change purely sensorial ; as in the various cases of " radiation of sensations" elsewhere alluded to (see SENS ATION) ; or, as in the sensations of nausea, of shuddering, of tickling, of pain, &c., which are frequently excited by changes purely mental. The degree of intensity, again, with which actual sensations are felt, depends as much upon the state of the mind as upon that of the corporeal organism. Thus, if we experience a slight itching in the skin, and direct our thoughts to it, we are speedily annoyed by its increase ; whilst, if we steadily fix our thoughts upon some other object, we are soon unconscious of the irritation. On the other hand, the complete absorption of the mind in some train of thought which engrosses its attention, may render the individual unconscious of impres sions that would ordinarily induce severe pain. This is remarkably seen in cases of natural and artificial somnambulism (see SLEEP) ; and it is probable that in many of the cases in which insane patients have in flicted severe wounds upon themselves, with out appearing to feel pain, the cause of the immunity from suffering is to be found in the entire possession which some dominant feel ing or idea has of their consciousness, so that they are not cognizant of any external im pressions but such as harmonize with it. Even in ordinary cases, it is well known that a severe injury suddenly inflicted, is much less felt at the moment than a far slighter injury of which the mind has been in expec tation ; thus, a limb has been carried away by a cannon-ball, or the chest traversed by a bullet, with far less consciousness of pain than is produced by the trivial incision made in ordinary venesection.