To every force there is an equal opposite force or reaction, and a reaction may be against a live body, but it is never suspected of being against the abstraction life or mind—that would indeed be enlarging the scope of mechanics!—the reaction is always against some other body. All stresses as a matter of fact occur in the ether; and they all have a material terminus at each end (or in exceptional cases a wave-front or some other recondite ethereal equivalent), that is to say something possessing inertia; but the timed or _opportune_ existence of a particular stress may be the result of organisation and control. Mechanical operations can be thus dominated by intelligence and purpose. When a stone is rolling over a cliff, it is all the same to "energy" whether it fall on point A or point B of the beach. But at A it shall merely dent the sand, whereas at B it shall strike a detonator and explode a mine. Scribbling on a piece of paper results in a certain distribution of fluid and production of a modicum of heat: so far as energy is concerned it is the same whether we sign Andrew Carnegie or Alexander Coppersmith, yet the one effort may land us in twelve months' imprisonment or may build a library, according to circumstances, while the other achieves no result at all. John Stuart Mill used to say that our sole power over Nature was to _move_ things; but strictly speaking we cannot do even that: we can only arrange that things shall move each other, and can determine by suitably preconceived plans the kind and direction of the motion that shall ensue at a given time and place. Provided always that we include in this category of "things" our undoubtedly material bodies, muscles and nerves. But here is just the puzzle: at what point does will or determination enter into the scheme? Contemplate a brain cell, whence originates a certain nerve-process whereby energy is liberated with some resultant effect; what pulled the detent in that cell which started the impulse? No doubt some chemical process: combination or dissociation, something atomic, occurred; but what made it occur just then and in that way? I answer, not anything that we as yet understand, but apparently the same sort of pre-arrangement that determined whether the stone from the cliff should fall on point A or point B—the same sort of process that guided the pen to make legible and effective writing instead of illegible and ineffective scrawls—the same kind of control that determines when and where a trigger shall be pulled so as to secure the anticipated slaughter of a bird. So far as energy is concerned, the explosion and the trigger-pulling are the same identical operations whether the aim be exact or random. It is intelligence which directs; it is physical energy which is directed and controlled and produces the result in time and space.