In the common notion of freedom the most universal characteristic seems to be the absence of external constraint, whether asserted to impel or to prevent. Where the immediate cause of a motion or change seems to lie in the thing which moves or changes and not in anything outside it, there, in a sense at all events, freedom is predicable. But this is a notion which is obviously not limited to human actions. Many of the movements and changes of animals and plants have their proximate causes in the things themselves; and the same might probably be said of any body in so far as it moved in accordance with Newton's second Law of Motion. It is thus we seem to talk of ‘free as air’, or of the wheels of a watch moving ‘freely’.

But there is an obvious defect in this wide notion, in that the limits, whether spatial or temporal, of any group we may take for our unit or thing, are always more or less arbitrary. A watch may be moving freely when its spring is driving it; but the movement of any one of its wheels is not free, because the wheel is driven by the spring or by another wheel. And, again, there seems no reason why we should single out the proximate or immediate cause for such preëminence, nor anything to determine how far back in the past a cause ceases to be proximate.