Cleo. That such improvements are impossible, is demonstrable; but if it was not, even then we could have little help from anatomy. The brain of an animal cannot be looked and searched into whilst it is alive. Should you take the main spring out of a watch, and leave the barrel that contained it standing empty, it would be impossible to find out what it had been that made it exert itself, whilst it showed the time. We might examine all the wheels, and every other part belonging either to the movement or the motion, and, perhaps, find out the use of them, in relation to the turning of the hands; but the first cause of this labour would remain a mystery for ever.
Hor. The main spring in us is the soul, which is immaterial and immortal: but what is that to other creatures that have a brain like ours, and no such immortal substance distinct from body? Do not you believe that dogs and horses think?
Cleo. I believe they do, though in a degree of perfection far inferior to us.
Hor. What is it that superintends thought in them? where must we look for it? which is the main spring?
Cleo. I can answer you no otherwise, than life.
Hor. What is life?
Cleo. Every body understands the meaning of the word, though, perhaps, nobody knows the principle of life, that part which gives motion to all the rest.
Hor. Where men are certain that the truth of a thing is not to be known, they will always differ, and endeavour to impose upon one another.
Cleo. Whilst there are fools and knaves, they will; but I have not imposed upon you: what I said of the labour of the brain, I told you, was a conjecture, which I recommend no farther to you than you shall think it probable. You ought to expect no demonstration of a thing, that from its nature can admit of none. When the breath is gone, and the circulation ceased, the inside of an animal is vastly different from what it was whilst the lungs played, and the blood and juices were in full motion through every part of it. You have seen those engines that raise water by the help of fire; the steam you know, is that which forces it up; it is as impossible to see the volatile particles that perform the labour of the brain, when the creature is dead, as in the engine it would be to see the steam (which yet does all the work), when the fire is out and the water cold. Yet if this engine was shown to a man when it was not at work, and it was explained to him, which way it raised the water, it would be a strange incredulity, or great dullness of apprehension, not to believe it; if he knew perfectly well, that by heat, liquids may be rarified into vapour.
Hor. But do not you think there is a difference in souls; and are they all equally good or equally bad?