Self interest, or more completely, individual self interest, is what is good for reason in its function of guiding the behavior of individual rational subjects. The individual self is the life, or four dimensional object, that the rational subject constructs during her cycle of reproduction, and what is in her individual self interest is what contributes to the natural perfection of the rational subject as an individual.
The necessary goals of individual (self) interest are goals the attainment of which controls conditions that affect the individual's reproduction as a multicellular animal.
Necessary goals of individual (self) interest include not only goals like those pursued by other animals, such as food and shelter, but also goals that must be controlled by rational subjects to be able to reproduce as parts of spiritual animals, such as social relations and the material means of leading a normal life. But reproduction itself is not a necessary goal.
Necessary goals of individual (self) interest are good for the rational subject because they contribute to the natural perfection of the rational subject as an individual organism. Thus, they are what ought to exist as far as reason is concerned. This means that values are facts, contrary to the usual assumption that the difference between facts and values preclude reducing values to facts.
This ontological explanation of what is good entails a naturalistic definition of "good," which is supposed to commit the so-called "naturalistic fallacy," according to G. E. Moore. But there is no such fallacy, because for someone who understands this ontological explanation of goodness and who recognizes herself as a rational being of the kind explained by it, it cannot be asked with significance, But is contributing to my natural perfection as an individual organism good?
This ontological explanation of what ought to exist is the kind of wisdom that Socrates was seeking, because it is knowledge about the nature of goodness that would make a rational being do what is good. Knowledge is virtue, because it explains not only what is good, but also why the good is good.
The way that goodness is explained by ontological philosophy is what makes it it possible to quell doubts about a naturalistic explanation of goodness. The overall structure of natural perfection makes it impossible for what contributes to it to turn out to be bad in some larger context, though the larger context may offer a more complete explanation of why the good is good.
There are optional goals of individual self interest, in addition to necessary goals. Optional goals are goals that are good for the individual because the indiviudal chooses to pursue them. But for a rational subject to choose them, they must already be good in some other way, that is, by contributing to the natural perfection of something other than the individual (or to artificial perfection).
Optional goals derive from the autonomy of reason. Reason enables the individual to do what is good simply because she believes that it is good, even when doing the good is opposed by strong immediate desires. And as a rational subject, she can often tell what is good for other things, because rational imagination is able to recognize natural perfection (that is, because of the rational interest in beauty, as well as truth and goodness).
The purusit of optional goals is good for reason because it contributes to the natural perfection of the rational being as an individual. Reason gives individuals the power to pursue goals beyond those that control conditions affecting their indiviudal reproduction, and the pursuit of optional goals makes individuals more powerful over their reproductive cycles. Since that is to do the most with one's lifetime of rational actions, it contributes to the natural perfection of the individual self.