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into three parts: first, we shall consider those powers which are a preamble to the intellect; secondly, the intellectual powers; thirdly, the appetitive powers.
Under the first head there are four points of inquiry:
(1) The powers of the soul considered generally;
(2) The various species of the vegetative part;
(3) The exterior senses;
(4) The interior senses.
FIRST ARTICLE [I, Q. 78, Art. 1]
Whether There Are to Be Distinguished Five Genera of Powers in the Soul?
Objection 1: It would seem that there are not to be distinguished five genera of powers in the soul--namely, vegetative, sensitive, appetitive, locomotive, and intellectual. For the powers of the soul are called its parts. But only three parts of the soul are commonly assigned--namely, the vegetative soul, the sensitive soul, and the rational soul. Therefore there are only three genera of powers in the soul, and not five.
Obj. 2: Further, the powers of the soul are the principles of its vital operations. Now, in four ways is a thing said to live. For the Philosopher says (De Anima ii, 2): "In several ways a thing is said to live, and even if only one of these is present, the thing is said to live; as intellect and sense, local movement and rest, and lastly, movement of decrease and increase due to nourishment." Therefore there are only four genera of powers of the soul, as the appetitive is excluded.
Obj. 3: Further, a special kind of soul ought not to be assigned as regards what is common to all the powers. Now desire is common to each power of the soul. For sight desires an appropriate visible object; whence we read (Ecclus. 40:22): "The eye desireth favor and beauty, but more than these green sown fields." In the same way every other power desires its