I have shown that the only way that desire is compatible with perfect happiness is if the desire and will to act are identical. But what is the relationship between happiness and goodness? Perfect happiness requires doing what is good. But does it require doing what is intrinsically good or not? Perfect happiness is incompatible with willing what is instrumentally good.
Perfect happiness is complete satisfaction with the current condition and future conditions. Instrumental goods are those goods that are chosen for the sake of another good. For example, money is useful because of what it can buy for you. No one desires money merely for its own sake. To demonstrate this imagine that someone was given a large sum of money but forbidden to spend any of it. The money would be worthless because the only value it has is instrumental.
Desire applies to what is good. But when something is desired it is desired insofar as we get what is intrinsically good. The example with the money shows this. Money is instrumentally good. But if a particular situation arises in which money cannot be used, then it is not desired either. Therefore, something is only desired if it is intrinsically good. What is instrumentally good is desired in those situations in which it is believed to gain what is intrinsically good, but it is only desired because of that.
This distinction means that the desire for an instrumental good cannot be identical to the will to act for that good. There is one key reason for this. When we act for an instrumental good, we act with the intent that we will do some intrinsic good or other. But that intrinsic good is not a part of what we are willing to do. Our desire is different. We desire the instrumental good only because it fulfills our desire for an intrinsic good. Since these are different, they cannot be identical. Since I have already shown that desire is only compatible with perfect happiness if the desire and will to act are identical, desiring instrumental goods is incompatible with perfect happiness.
Since desiring instrumental goods is incompatible with perfect happiness, but intrinsic goods are compatible with perfect happiness. When we desire something that is intrinsically good, we desire what we act to achieve. This means that if I desire to learn something just for the sake of learning it, then my act of learning it is exactly what I had desired. Apart from external circumstances, there is no reason that the two could not be identical. Therefore, intrinsic goods are compatible with perfect happiness.