Page images
PDF
EPUB

the real external cause of our sensations, may be different from them, in every respect. If, then, the materialist mean, that the sentient principle resembles our ideas, the proposition is nugatory; as it only states, that the sentient principle resembles itself: but, if he mean, that the sentient principle resembles the cause of our ideas, he asserts, that what we know resembles that, which we do not know.'

In the first section of the book, Mr. Brown considers Dr. Darwin's theory of Sensorial Power. He observes that, according to Dr. Darwin's own statement, the original production of sensorial power, which is supposed in the Zoonomia to be secreted by the brain and spinal marrow, must be impossible ; since it is necessary to its secretion, that the gland should previously possess a quantity of sensorial power, and should be excited to action by its specific stimulus. He thus reduces Dr. Darwin's opinion to the absurdity of stating, that the power exists previously to its own existence.

• This objection,' he says, (p. 2,) it will perhaps be urged, is of little weight, if we suppose the embryon, when originally secreted, to have been complete in its structure, and a small quantity of sensorial power to have existed in its minute brain.

But the ena bryon, according to Dr. Darwin, is a simple filament, without sen. sorial power, or the means of producing it; and though we should admit, even in these circumstances, the possibility of the gradual formation of a gland, the fibres of the gland, not possessing the source of animal motion, must for ever remain inactive.'

Mr. B. next examines Dr. Darwin's supposition that the oxygen, received into the system by respiration, supplies the material for the production of sensorial power, or the spirit of animation; and he shews, from the revival of persons in whom the phänomena of life had been interrupted by drowning, and of animals which had passed the winter in a torpid state, the difficulty of allowing that oxygen, which demands so quick a supply of the substance affording it in respiration, should suddenly change its nature, and become stationary in the fibres of the system.

The author now proceeds to shew (p. 8, & seq.) that, in the animal kingdom, there is an extensive class of animals, which have no brain ;' and that in those instances, therefore, Dr. D. must admit the existence of irritability without sensorial power ; of a quality, without a substance.

He then examines, at considerable length, the various properties ascribed by Dr. Darwin to the spirit of animation. A dextrous metaphysician finds it easy to charge contradictions on any theory, which attempts to explain the particular modi. fication on which the phænomena of life depend. Where it is so practicable to object, and so difficult to defend the subject,

we cannot avoid praising the gallantry of the author of Zoonomia, in his attempt to take a new position ; and if we must, eventually, concede the victory to his antagonist, we may still say of him;

Magnis tamen excidit ausis. We are, indeed, cruelly situated, with philosophers who admit neither the existence of matter nor of spirit. Our author, for example, denies that extension is an essential quality of matter, and defines it to be nothing more than number (p. 28, note). In such cases, we must surely alter our voca. bulary; or substitute, for the names of uncertain and unknown things, some arbitrary sign, such as that of the negative quantity in Algebra ; otherwise, the confusion of terms will become an insuperable evil in metaphysical reading.

In justice to Dr. Darwin, however, we must observe that the reductio ad absurdum, in Mr. Brown's first argument, is by no means inevitable. Dr. D. clearly supposes that sensorial power is communicated from the parent to the embryo, in the moment of production ; and the continuance of this communication is provided by the supply of blood from the mother. This explanation obviates the whole difficulty started by Mr. Brown.

His arguments against the doctrine of sensorial power are more conclusive, when he undertakes to shew that, granting such a fluid to exist, it would exhibit phænomena very different from those of the human system :

• If sensorial power possess a tendency to equilibrium, the partial accumulation or diminution of it, by exertion or repose, is impossible. Hence, after remaining long in the dark, and returning suddenly to the light, there should be no sensation of dazzling in the eyes, when sufficient time has elapsed, for the contraction of the iris; because the sensorial power, which would have occasioned pain, if wholly accumulated in the retinal tibres, is distributed through the sensorium, so as to render the accumulation in the eye inconsiderable. On the same principle, the arm, which is at rest, should share the fatigue of that, which is exercised : the want of sensorial power, and consequent languor, should be equally felt by the n:ost distant fibre; nor should any musele cease to be capable of exertion, till universal debility be introduced into the system. It will, perhaps, be thought, that the general accumulation, or diminution of sensorial power, during the action, or maction of certain muscles, may be sufficient to produce the cllects, observed in the particular organs. But, we find, that, after taking less than our usual food, or exercise, the quantity, thus accu. mulated in the eye, is not sufficient to produce the painful sensation of dazzling, when excited by the usual stimulus of light.

« The mere existence of sensorial power, its capacity of producing fibrous inotion, and the derivation of it from the brain, to the most remote organ of the system, are not alone necessary to the truth of

Dr.

Dr. Darwin's theory. It must also be proved, that sensorial power is expended during exertion, and that the expenditure is proportional to the contraction. These, however, it will be shown in the section on stimulus and exertion, Dr. Darwin has taken for granted, though, from the nature of exertion, on his own principles, no reason of the supposition can be assigned.'

The author then proceeds to deduce, from Dr. Darwin's premises, the existence of a multitude of distinct beings in each individual, as irritation, sensation, volition, and association, are essential qualities of the most minute portion of sensorial power;' and he concludes the section with these words: • If particles of sensorial power be indeed concerned, in the operations of life, they are only secondary agents. There is one mind which governs the various parts of our complicated frame:

- One diffusive soul Wields the large limbs, and mingles with the whole.' In the ad Section, which treats of the faculties of the sensorium, the author objects to the supposed modifications of the sensorial power, as incompatible with the qualities of a material Auid. "Sensorial power exists in the system, in a certain state, before the first irritation. In this state, it must for ever continue; and the phenomena of life, depending on the possibility of a change of the mode of affection of the vital principle, cannot, therefore, be explained by the supposed exista ence of a principle, essentially immutable in its qualities.' On this idea, he has enlarged with great ingenuity.

Henext shews that the four modifications of this power which Dr. Darwin has assigned, irritation, sensation, volition, and association, instead of admitting the distinctions pointed out by him, must, on his own principles, be one and the same. On the subject of irritation, Mr. Brown takes occasion to consider the very extraordinary opinion advanced by Dr. Darwin, that our ideas of figure result from the actual impression of a similar figure on the sensorium; that we perceive the idea of a square, for example, because the figure of a square is then really delineated in the sensorium :

A square surface, pressed on the palm of my hand, occasions a square configuration of the fibres, and, with them, of the sensorial power. This configuration, however, is not the irritative idea of figure, but the stimulus, which excites irritation. If, therefore, irii. tation terminate in the contraction of fibres, the sensorial

power must lose its similarity to the compressing body, and the idea excited be that of a different figure, as of'a circle, or a triangle. But the idea of the square continues: the configuration, therefore, continues ; and irritation is not, as Dr. Darwin supposes, an exertion, or change of the spirit of animation, exciting the fibres to contraction.'

It is worth remarking, that this theory of perception is very similar to the Cartesian doctrine of perception and memory i and we are surprised that the resemblance has not been more generally noticed. Des Cartes applied this theory, very ingeniously, to solve several questions respecting memory. Those figures which were most deeply impressed in the brain were the latest in being worn out: but, by. length of time, all were obliterated, excepting those which had been repeatedly retraced and strengthened; and as the brain, according to the philosophy of that time, became harder and drier in old age, it was said to be less fitted to receive permanent impressions. A system of anatomy was published, during the reign of Cartesianism, in which the progress of memory was exhibited in figures, on this plan. The difficulty of comprehending the nature of perception was certainly not relieved, by adding this gratuitous supposition to its real phænomena.

In pursuing the subject of these modifications of sensorial power, we meet with some excellent observations on the danger of using words already known, in a new sense. Our readers, we are sure, will thank us for extracting this passage:

“ The words idea, perception, sensation, recollection, suggestion, and association,” it is observed, in the preface to Zoonomia, “ are each of them used in this treatise in a more limited sense than in the writers of metaphysic. The author was in doubt, whether he should rather have substituted new words instead of them; but was at length of opinion, that new definitions of words already in use, would be less burthensome to the memory of the reader.” It is much to be regretted, that this mode is ever followed : for, though words already in use be less burthensome to the memory, the advantage is more than counterbalanced by the greater difficulty of remembering their new definitions. A train of reasoning can then only be accurately understood, when the terms suggest uniformly the same ideas. But, when different ideas are expressed by the same sign, the mind insensibly passes from one to the other, and the proposition, to which the reader assents, is frequently different from that, which the language of the authors was intended to convey. If our reasoning he thus subject to confusion, when the sign is equally associated with two ideas, the difficulty must be proportionally greater, when the foreign idea is more readily suggested; and this must always be the case, when new definitions of old terns are adopted. The former idta has all the force of the original association ; in our trains of thought, it has been invariably conjoined with the sign; and it recurs spontaneously to the mind, when the chai...ters are perceivedl, But the ties of the new association are feeble ; and we are frequently obliged to retrace the definition, to be convinced, that we have not mistaken its meaning. With what labour of mind, should we peruse a treatise on colours, in which blue and yellow, red and green,

orange

orange and violet, were mutually substituted! Yet, when new terms are used, as when we read a treatise on colours, in a foreign language, we follow the author without difficulty. Nor is it only to the reader, that this mode of innovation is productive of confusion. The author himself, however strongly he may have connected the new idea with the sign, is still under the influence of prior habits; and will thus less readily discover an error in his reasoning, when the propositions are just, in the former signification of the terms. It is this ambiguity, which has deceived Dr. Darwin, in classing the phenomena of mind.

• Pleasure, and pain, are considered in Zoonomia, in two points of view, eitheir simply as phenomena, or as the causes of phenomena. It is only in the latter sense, to pleasure, or pain, when causing fibrous motions, that Dr. Darwin gives the name of sensation ; and the reader is earnestly entreated by him to keep the distinction in his mind, p. 12. All those sensorial motions, therefore, which do not terminate in exciting the muscles, or organs of sense, are excluded from his system, as they are not irritations, sensations, volitions, nor associations; and among these the greater number of our pleasures and pains must be classed : yet, in many passages of Zoonomia, the original limitation of the term seems to have been forgotten, and sensation to have been used, as synonymous with pleasure, and pain.'

Mr. Brown next points out several phænomena of life, which cannot be reduced to any of Dr. Darwin's four classes of sensorial motions; and he traces, in a very striking manner, the want of precision in the Zoonomia, arising from the adaptation of new definitions to common words.

In the third Section, (p. 70,) the author considers the Classes of Fibrous Motions ; following the arrangement of the Zoonomia. Dr. D. has supposed that all fibrous contractions were originally caused by the irritations of external objects : but that painful or pleasurable sensations often accompanying those irritations, the contractions became exciteable by those sensations; and that, as efforts of the will accompanied those sensations, the contractions were at length, by habit, causable by volition alone. To this doctrine, Mr. Brown objects that, whatever may

be attributed to the power of habit, it is impossible that the reversed habit, here supposed, should produce the effects ascribed to it; that a person, for example, should repeat the alphabet backward, with ease, because he cannot repeat it in the usual order. He adds that, even according to Dr. Darwin's own statement, the cause of fibrous contractions must be uniformly resolved into irritation.

In the fourth Section, of Stimulus and Exertion, the author displays great ingenuity, in combating Dr. D.'s opinion that sensorial power is expended on every sensorial change :

· That we have no reason to consider the spirit of animation, as expended during exertion, will be evident, if we attend to the nature

« PreviousContinue »