A Fifth Cardinal Virtue?

Wisdom, temperance, courage, justice, and piety--are these five names for the same thing, or is there underlying each of these names a unique thing, a thing with its own power or function, each one unlike any of the other?

sofi/a kai\ swfrosu/nh kai\ a)ndrei/a kai\ dikaiosu/nh kai\ o(sio/thj, po/teron tau=ta, pe/nte o)/nta o)no/mata, e)pi\ e(ni\ pra/gmati/ e)stin, h)\ e(ka/stw| tw=n o)noma/twn tou/twn u(po/keitai/ tij i)/dioj ou)si/a kai\ pra=gma e)/xon e(autou= du/namin e(/kaston, ou)k o)\n oi(=on to\ e(/teron au)tw=n to\ e(/teron;

It's the question asked by Socrates of Protagoras (349b), of course. What strikes me as especially interesting, however, is the addition of 'piety' to Plato's usual list of four chief virtues.

Why is this interesting? In the Protagoras, Plato is clearly testing the idea that all virtues are forms of 'wisdom' or 'knowledge' (332a-333b), and that all are forms of justice (329d-332a). One might even take the hedonic calculus at the dialogue's end to be expressing the view that all virtuous action is a matter of appropriately discounting present pains ('courage') or pleasures ('temperance'). So, then, is Plato's view, similarly, that all the virtues are forms of piety? That would seem to be the view of the dialogue: it seems presumed, for instance, in the way Socrates refers to piety at 330e.

But, then, is this Plato's view, also, outside of the Protagoras, and, if so, what are the implications of this for his understanding of virtue? I've done no careful study. I would simply note that one finds evidence in the Crito that he construes other virtues as forms of piety. There Socrates explains that standing firm at one's post in battle, even if this means giving up one's life, is a high expression of piety towards one's country:

You must persuade your country or obey its orders, and endure in silence whatever it instructs you to endure, whether blows or bonds, and if it leads you into war to be killed, you must obey. To do so is right, and one must not give way or retreat or leave one's post, but both in war and in courts and everywhere else, one must obey the commands of one's city and country, or persuade it as to the nature of justice (51b, and cp. Phaedo 62b, Apology 28d).

Of course remaining at one's post, even to the point of death, was the paradigm of courage.

What would the implications of this be? Roughly, it would follow that it is misguided to construe virtue, for Plato, as an expression of autonomous human reason (as Vlastos does, in "Socratic Piety", in fact). Piety is the relationship of undischargable debt, and therefore obedience, to superior authorities, such as parents, ancestors, the city, and the gods, who have bestowed upon us the highest goods (life, sustenance, education). Xenophon reports that Socrates viewed human life as under the providential care of the gods; in the Phaedo, Socrates explains that he was attracted to Anaxagoras precisely because he was hoping to find corroboration for the idea that a divine power orders everything to the best end; Plato in Laws 10 regards recognition of the beneficial superintendence of the gods as the indispensable basis for an orderly and virtuous society.

But virtue cannot be the autonomous exercise of human reason, if a fundamental moral attitude, or the most fundamental attitude, is a willingness to subordinate oneself to the authority of a higher intelligence.