VII.  There are two ways by which we become acquainted with the passions of others:  first, by having observed the effects of them, as of fear or anger, on our own bodies, we know at sight when others are under the influence of these affections.  So when two cocks are preparing to fight, each feels the feathers rise round his own neck, and knows from the same sign the disposition of his adversary:  and children long before they can speak, or understand the language of their parents, may be frightened by an angry countenance, or soothed by smiles and blandishments.

Secondly, when we put ourselves into the attitude that any passion naturally occasions, we soon in some degree acquire that passion; hence when those that scold indulge themselves in loud oaths, and violent actions of the arms, they increase their anger by the mode of expressing themselves:  and on the contrary the counterfeited smile of pleasure in disagreeable company soon brings along with it a portion of the reality, as is well illustrated by Mr. Burke. (Essay on the Sublime and Beautiful.)

This latter method of entering into the passions of others is rendered of very extensive use by the pleasure we take in imitation, which is every day presented before our eyes, in the actions of children, and indeed in all the customs and fashions of the world.  From this our aptitude to imitation, arises what is generally understood by the word sympathy so well explained by Dr. Smith of Glasgow.  Thus the appearance of a cheerful countenance gives us pleasure, and of a melancholy one makes us sorrowful.  Yawning and sometimes vomiting are thus propagated by sympathy, and some people of delicate fibres, at the presence of a spectacle of misery, have felt pain in the same parts of their own bodies, that were diseased or mangled in the other.  Amongst the writers of antiquity Aristotle thought this aptitude to imitation an essential property of the human species, and calls man an imitative animal. [Greek:  To zoon mimomenon].

These then are the natural signs by which we understand each other, and on this slender basis is built all human language.  For without some natural signs, no artificial ones could have been invented or understood, as is very ingeniously observed by Dr. Reid. (Inquiry into the Human Mind.)

VIII.  The origin of this universal language is a subject of the highest curiosity, the knowledge of which has always been thought utterly inaccessible.  A part of which we shall however here attempt.

Light, sound, and odours, are unknown to the foetus in the womb, which, except the few sensations and motions already mentioned, sleeps away its time insensible of the busy world.  But the moment he arrives into day, he begins to experience many vivid pains and pleasures; these are at the same time attended with certain muscular motions, and from this their early, and individual association, they acquire habits of occurring together, that are afterwards indissoluble.