‘The one set of machines are characterised by their calculability—the other by their incalculability. The one set, when at work, are not easily put wrong, while the other set are characterised by great delicacy of construction.’ 180. Having thus defined the two kinds of machines, let us now see to what extent a living being may be regarded as a machine, and also to which of these two categories he belongs. What our machines enable us to do is merely to transform energy. Our readers are well aware, by what we have already said (Art. 102), that it is just as impossible to create energy as it is to create matter. Thus a clock has to be wound up before it will go; an engine has to be stoked with coal; a rifle or cannon has to be charged with powder; and in short, all machines, whether delicately constructed or not, whether calculable or incalculable, are merely transmuters of energy and not creators of it. To this law the living being is no exception. The creatures of this world (and it is of such we are now speaking) are certainly not creators of energy; but in respect of the great law of the conservation of energy, such beings must be regarded in the very same light as any other machines. But there is yet another analogy between living beings and inanimate machines. When we study the working of any machine, we find that each transformation of energy brought about has a material antecedent; the effect produced has a cause from which it springs, and this cause is one which we are probably able to recognise from our knowledge of the laws of matter. To take an example: in a steam-engine the amount of work produced depends upon the amount of heat carried from the boiler to the condenser; and this amount depends in its turn upon the amount of coal which is burned in the furnace of the engine. In like manner, the velocity of the bullet which issues from a rifle depends upon the transformation of the energy of the powder; this in its turn depends upon the explosion of the percussion cap; this again upon the fall of the trigger; and lastly this upon the finger of the man who fires the rifle. Now, without attempting to define what life is, and leaving all speculations regarding it to our last chapter, we yet think it may safely be said that a living being is analogous to a machine in this particular also. Let us take the man who fires the rifle. We can trace back the motion of his forefinger to the contraction of a muscle; and we can go even further back and connect this contraction with a stimulus sent along the nerves from the brain, so that a material effect is here seen to be brought about by a material antecedent, just as truly as in an inanimate machine. Indeed, we may generalise, and say that, _so far as we can physically investigate a living being_, we may take it for granted that a material effect is due to a strictly material antecedent in his case also.