The soul according to the Thomists, who use the Aristotelian definition, is the first entelechy of a natural organic body that has life in potency.[41] It is the determination that gives the body its specific and substantial being; the primal actuation of a body or matter, since only in matter is there a distinction between potency for substantial being and substantial actuality. An entelechy is a realization, actuality, full perfection; sight, for example, is the entelechy of the eye. This body is natural, not merely instrumental; it is energized by an immanent principle, not moved by an external force like a tool. The body is also organic; it must have organs, faculties, parts destined to perform definite functions. To say the entelechy has life in potency means that since life, or the operation of the soul, is an immanent act, there must be a receptacle within which it can be immanent, and the soul is the primal actualization of that organic body, which is in potency to produce those immanent actions in which life consists. A body might be in potency while it still has no principle of operation, or, secondly, while it has such a principle but is not using it. In the second condition the human body is in potency for life at the moment of actualization. [41] ἡ ψυχή ἐστιν ἐντελέχεια ἡ πρώτη σώματος ψυσικοῦ ὀργανικοῦ δυνάμει ζωὴν ἔχοντοσ (_De Anima_, ii, 1). A form fixes a thing in its prχοντοσoper species, and the rational soul is such a form for the human body. This substantial form is the completion, perfection, in operability and existence, of the matter that receives it. It is the formal cause of man, not the efficient cause, although it is the efficient cause of subsequent vital operations. An efficient cause makes something numerically different from itself by its own real and physical action; a formal cause and a material cause do not make anything different from themselves numerically, but they intrinsically constitute the effect--they are intrinsic causes. The human soul as the substantial form virtually contains vegetative and sensory faculties, and through these lower organic capacities it informs and animates the body. That form, together with the matter, the body, does the vital acts of the composite human nature. The rational soul enters the body at the beginning, and first uses its vegetative faculty until the fetus is far enough advanced to be a subject for the action of the sensory faculty of the soul. Later, some time after the birth of the child, when the body is sufficiently formed, the intellectual faculty comes into use.