T 2.1.12.2

'Tis usual with anatomists to join their observations and experiments on human bodies to those on beasts, and from the agreement of these experiments to derive an additional argument for any particular hypothesis. 'Tis indeed certain, that where the structure of parts in brutes is the same as in men, and the operation of these parts also the same, the causes of that operation cannot be different, and that whatever we discover to be true of the one species, may be concluded without hesitation to be certain of the other. Thus tho' the mixture of humours and the composition of minute parts may justly be presum'd to be somewhat different in men from what it is in mere animals; and therefore any experiment we make upon the one concerning the effects of medicines will not always apply to the other; yet as the structure of the veins and muscles, the fabric and situation of the heart, of the lungs, the stomach, the liver and other parts, are the same or nearly the same in all animals, the very same hypothesis, which in one species explains muscular motion, the progress of the chyle, the circulation of the blood, must be applicable to every one; and according as it agrees or disagrees with the experiments we may make in any species of creatures, we may draw a proof of its truth or falshood on the whole. Let us, therefore, apply this method of enquiry, which is found so just and useful in reasonings |concerning the body, to our present anatomy of the mind, and see what discoveries we can make by it.