An instrument is any object which makes possible the attainment of desired ends. It is not essential to the being of the instrument that it be inanimate, worked over, or its mode of operation understood. A pigeon can carry a message; a stone as well as a hammer can be employed to break a glass; a child can start an automobile. The instrument need not function in the interests of men, be set into operation by living beings, or even be used at all. Leaves serve some insects as umbrellas do men; the rays of the sun may turn a screw; a new knife has not yet had a chance to cut. A cow is an instrument for the production of milk, a hen for the production of eggs, though neither is inanimate, worked over, nor its operation understood; they can serve the interests of other animals, are able to initiate their own movements, and may not yet have had occasion to produce the desired results. But they are instruments because they are acknowledged means for the production of the desired milk and eggs.