From Arrian (Epictetus), Discourses, 1.27:
“…unless piety and self-interest be conjoined, piety cannot be maintained in any man.”
Self-interest must converge with the loftiest conception of the human found in the conception of the human as united with the divine. So, piety. Real self-interest is different from selfishness, greed. It is not mere want. It is the combination of want and should. Self-interest is acting in way that is best for the self acting.
Want is natural in the human animal. And, the appearance of self-interest is natural in the human animal. It must be presupposed about each human-animal in order to interpret change as action at all. Action cannot be understood unless it is understood as the effort to attain an end which is good (that which is to be moved toward). So, each action is by definition subjectively considered to be self-interested.
Why must it be combined with piety? Not merely to enrich the pastors, priests, imams. No, forget the social club and its elected president. Let us consider piety as the proper attention and attitude to the unity of everything (the god). The organization of attention, attitude and action that considers the organization from the singular unifying standpoint is what piety involves. To consider the question of piety is to ask: “If I were to look in on my life from the omnipotent, omniscient and beneficent perspective, how would I organize myself?” The truly pious has read the script of the divine logos and has instantiated that message. Hearing and obeying are one.
Consideration of the unity of everything and the integration into subjectivity of the objective is deeply in self-interest. It is in the interest of the self to adapt to the conditions in which it finds itself. To begin from the most unified standpoint and work back to the most immediately contextualized standpoint is to invite the most determinative factors into consideration thus increasing the probability of successful adaptation.
How to move toward the unity of everything? I find a correlation between leaving self dissatisfied and sensitivity to such unity. I do not mean to make the mistake of pursuing self-mortification as Buddha warns against. There is nothing in pain qua pain that moves one closer. What I usually need, to use a sound metaphor, is to silence the noice coming from me so that I can hear what is not-me. To continue the metaphor, those voices of want and demand do not go away voluntarily and willingly. In fact, ignore the want and demand has a way of increasing its effort to be noticed. The noise grows louder in the near-term. But, the wisdom of the sages suggests that there is a point, and it is both more difficult and further away than the subjective hoping for unity with the everything wants, a point at which the voices grow weaker. As those voices grow weaker, voices of subjective immediate want, two things happen: first, the strength of the subjectivity which is not aligned with any particular subjectivity grows stronger. Second, other not-me voices of very different sorts–voices of creeks, voices of trees, voices of stars, voices of gum-smacking-freshman-heavy-eye-lidded student, voices of screens that are not the words on the page, not the sounds in the videos, but a different what-it-means, becomes possible.
That the immediate clamor of the mob of subjective want is quieted does not immediately imply the crowning of any sagacious rule. But, generally, it does move the dial in that direction. Piety and self-interest must be unified.