I answer that, The will is moved in two ways: first, as to the exercise of its act; secondly, as to the specification of its act, derived from the object. As to the first way, no object moves the will necessarily, for no matter what the object be, it is in man's power not to think of it, and consequently not to will it actually. But as to the second manner of motion, the will is moved by one object necessarily, by another not. For in the movement of a power by its object, we must consider under what aspect the object moves the power. For the visible moves the sight, under the aspect of color actually visible. Wherefore if color be offered to the sight, it moves the sight necessarily: unless one turns one's eyes away; which belongs to the exercise of the act. But if the sight were confronted with something not in all respects colored actually, but only so in some respects, and in other respects not, the sight would not of necessity see such an object: for it might look at that part of the object which is not actually colored, and thus it would not see it. Now just as the actually colored is the object of sight, so is good the object of the will. Wherefore if the will be offered an object which is good universally and from every point of view, the will tends to it of necessity, if it wills anything at all; since it cannot will the opposite. If, on the other hand, the will is offered an object that is not good from every point of view, it will not tend to it of necessity. And since lack of any good whatever, is a non-good, consequently, that good alone which is perfect and lacking in nothing, is such a good that the will cannot not-will it: and this is Happiness. Whereas any other particular goods, in so far as they are lacking in some good, can be regarded as non-goods: and from this point of view, they can be set aside or approved by the will, which can tend to one and the same thing from various points of view.