The same impression arises when we compare the brain of man with that of the animals. The difference at first appears to be only a difference of size and complexity. But, judging by function, there must be something else besides. In the animal, the motor mechanisms that the brain succeeds in setting up, or, in other words, the habits contracted voluntarily, have no other object nor effect than the accomplishment of the movements marked out in these habits, stored in these mechanisms. But, in man, the motor habit may have a second result, out of proportion to the first: it can hold other motor habits in check, and thereby, in overcoming automatism, set consciousness free. We know what vast regions in the human brain language occupies. The cerebral mechanisms that correspond to the words have this in particular, that they can be made to grapple with other mechanisms, those, for instance, that correspond to the things themselves, or even be made to grapple with one another. Meanwhile consciousness, which would have been dragged down and drowned in the accomplishment of the act, is restored and set free.[77] The difference must therefore be more radical than a superficial examination would lead us to suppose. It is the difference between a mechanism which engages the attention and a mechanism from which it can be diverted. The primitive steam-engine, as Newcomen conceived it, required the presence of a person exclusively employed to turn on and off the taps, either to let the steam into the cylinder or to throw the cold spray into it in order to condense the steam. It is said that a boy employed on this work, and very tired of having to do it, got the idea of tying the handles of the taps, with cords, to the beam of the engine. Then the machine opened and closed the taps itself; it worked all alone. Now, if an observer had compared the structure of this second machine with that of the first without taking into account the two boys left to watch over them, he would have found only a slight difference of complexity. That is, indeed, all we can perceive when we look only at the machines. But if we cast a glance at the two boys, we shall see that whilst one is wholly taken up by the watching, the other is free to go and play as he chooses, and that, from this point of view, the difference between the two machines is radical, the first holding the attention captive, the second setting it at liberty. A difference of the same kind, we think, would be found between the brain of an animal and the human brain.