“What is essential in the machine is only the interaction of its parts. The unifying principle that governs that interaction does not exist in the object itself but outside it as a plan in the head of its builder. Only the most extreme shortsightedness can deny that the difference between an organism and a mechanism is precisely the fact that in a machine the determining principle governing the interrelationship of its parts is external (and abstract), whereas in an organism it assumes a real existence in the object itself. Thus, the sense—perceptible conditions of an organism do not appear merely to follow one from another, but are governed by an inner principle that is imperceptible to the senses. In this sense this principle is no more perceptible to the senses than the plan in the builder's head, which is also present only to the mind. Essentially, it is such a plan, except that it has entered the organism's inner being and affects it directly, not through a third party, the builder.”