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 affections, in men; it will be reasonable to suppose, that they will also be sufficient to solve the analogous phænomena in brutes. And, conversely, it seems probable, that an endeavour to apply and adapt these doctrines to brutes will cast some light and evidence upon them, as they take place in men. And thus the laws of vibrations and association may be as universal in respect of the nervous systems of animals of all kinds, as the law of circulation is with respect to the system of the heart and bloodvessels; and their powers of sensation and motion be the result of these three laws, viz. circulation, vibrations, and association, taken together. These three laws may also be most closely united in their ultimate cause and source, and flow in all their varieties from very simple principles. At least this is the tenor of nature in many similar cases.

As the whole brute creation differs much from, and is far inferior to man in intellectual capacities; so the several kinds of animals differ much from each other in the same respect. But I shall, in this Section, confine myself chiefly to the consideration of the first difference, viz. of that between mankind and the brute creation in general; and endeavour to assign such reasons for it, as flow from, or are agreeable to, the theory of these papers. We may suppose then, that brutes in general differ from, and are inferior to man, in intellectual capacities, on the following accounts:

First, The small proportional size of their brains.

Secondly, The imperfection of the matter of their brains, whereby it is less fitted for retaining a large number of miniatures, and combining them by association, than man’s.

Thirdly, Their want of words, and such like symbols.

Fourthly, The instinctive powers which they bring into the world with them, or which rise up from internal causes, as they advance towards adult age.

Fifthly, The difference between the external impressions made on the brute creation, and on mankind.

First, then, As the brains of brutes are less in proportion to the bulk of the other parts, than those of men; and as the internal parts of the brain appear from these papers to be the peculiar seat of ideas, and intellectual affections; it seems very natural to expect, that brutes should have a far less variety of these than men. The parts which intervene between the optic and auditory nerves, being proportionably less, for instance, in brutes, will not admit of so great a variety of associations between the several ideas of these senses, because the optic and auditory nerves cannot have so great a variety of connexions and communications with each other.

To this it is to be added, that the internal parts belonging to the olfactory nerves, and, perhaps, those belonging to the nerves of taste, take up, probably, a greater proportional part of the medullary substance of the brain than in us, since most brutes