Objection 1. It would seem that the goodness of the will does not depend on the object alone. For the end has a closer relationship to the will than to any other power. But the acts of the other powers derive goodness not only from the object but also from the end, as we have shown above (18, 04). Therefore the act also of the will derives goodness not only from the object but also from the end.
Objection 2. Further, the goodness of an action is derived not only from the object but also from the circumstances, as stated above (Question 18, Article 3). But according to the diversity of circumstances there may be diversity of goodness and malice in the act of the will: for instance, if a man will, when he ought, where he ought, as much as he ought, and how he ought, or if he will as he ought not. Therefore the goodness of the will depends not only on the object, but also on the circumstances.
Objection 3. Further, ignorance of circumstances excuses malice of the will, as stated above (Question 6, Article 8). But it would not be so, unless the goodness or malice of the will depended on the circumstances. Therefore the goodness and malice of the will depend on the circumstances, and not only on the object.
On the contrary, An action does not take its species from the circumstances as such, as stated above (18, 10, ad 2). But good and evil are specific differences of the act of the will, as stated above (Article 1). Therefore the goodness and malice of the will depend, not on the circumstances, but on the object alone.
I answer that, In every genus, the more a thing is first, the more simple it is, and the fewer the principles of which it consists: thus primary bodies are simple. Hence it is to be observed that the first things in every genus, are, in some way, simple and consist of one principle. Now the principle of the goodness and malice of human actions is taken from the act of the will. Consequently the goodness and malice of the act of the will depend on some one thing; while the goodness and malice of other acts may depend on several things.
Now that one thing which is the principle in each genus, is not something accidental to that genus, but something essential thereto: because whatever is accidental is reduced to something essential, as to its principle. Therefore the goodness of the will's act depends on that one thing alone, which of itself causes goodness in the act; and that one thing is the object, and not the circumstances, which are accidents, as it were, of the act.
Reply to Objection 1. The end is the object of the will, but not of the other powers. Hence, in regard to the act of the will, the goodness derived from the object, does not differ from that which is derived from the end, as they differ in the acts of the other powers; except perhaps accidentally, in so far as one end depends on another, and one act of the will on another.
Reply to Objection 2. Given that the act of the will is fixed on some good, no circumstances can make that act bad. Consequently when it is said that a man wills a good when he ought not, or where he ought not, this can be understood in two ways. First, so that this circumstance is referred to the thing willed. And thus the act of the will is not fixed on something good: since to will to do something when it ought not to be done, is not to will something good. Secondly, so that the circumstance is referred to the act of willing. And thus, it is impossible to will something good when one ought not to, because one ought always to will what is good: except, perhaps, accidentally, in so far as a man by willing some particular good, is prevented from willing at the same time another good which he ought to will at that time. And then evil results, not from his willing that particular good, but from his not willing the other. The same applies to the other circumstances.
Reply to Objection 3. Ignorance of circumstances excuses malice of the will, in so far as the circumstance affects the thing willed: that is to say, in so far as a man ignores the circumstances of the act which he wills.
Source: Summa Theologica (New Advent)