This saying recognizes the fact that "a material system can do no work on itself"; but needs external aid.

When a man pulls upward on his boot straps, the upward force that he exerts is exactly balanced by the downward reaction exerted by his boot straps; but when he jumps, the downward thrust of his legs causes an equal reaction of the earth, which exerts a direct force upward upon the man; and it is this external force that moves him over the fence.

It is this external force, the reaction of the earth or air or water, which moves every animal that walks, or bird that flies, or fish that swims.

It is the will of the Almighty, acting through the various stimuli of nature, that causes the desire to walk, and all the emotions and actions of men.

If He shall cause any new force to act on men, their line of conduct will surely change.