As the SEP article linked above points out, the concept of supererogation has received little attention in ethical philosophy until recently. An interesting exception is the Protestant response to the Catholic acceptance of supererogatory actions. My favorite statement comes from Article 14 of the Anglican Thirty-Nine Articles of Religion. Here is the relevant text:

Voluntary Works besides, over and above, God’s Commandments, which they call Works of Supererogation, cannot be taught without arrogancy and impiety: for by them men do declare, that they do not only render unto God as much as they are bound to do, but that they do more for his sake, than of bounden duty is required: whereas Christ saith plainly When ye have done all that are commanded to you, say, We are unprofitable servants.

I like this negative take on supererogatory actions. First, I think it is a clever application of scripture to a philosophical issue. Second, I think it is quite correct to say that in fact the demands of God are no lower than human capability. Many of the specific imperatives in scripture (e.g. love your enemies; bless those who persecute you; run two miles instead of one) describe actions that are normally considered supererogatory. And the general thrust of the morality in the gospel (Be perfect as your heavenly father is perfect) clearly leaves no room for supererogation.

We might distinguish between denying supererogation in practice and denying it in principle. Article 14 could be read as merely denying it in practice – human beings are just not capable of exceeding duty, even though duty could in principle be exceeded. This view is open to fairly simple counterexamples. Even if a human life cannot achieve a net amount of virtue exceeding the demands of duty (say, being morally perfect), a human life could exceed the demands of duty in a particular case (say, mowing your neighbor’s lawn for free). So even though we might be “unprofitable servants” overall, this won’t necessarily mean we never exceed duty.

But I would read Article 14 as saying that there simply aren’t supererogatory actions available to us. Mowing your neighbor’s lawn for free, if it is a good thing to do, is obligatory. Surely this move is consistent to the stringent demands of the Sermon on the Mount. We might ask, what about mowing your neighbor’s lawn for free plus giving him a glass of water? Or mowing his lawn, giving him a glass of water, and saying something nice? But there would be a limit to what is good to do for your neighbor, especially in the context of competing obligations (say, to your family). At some point, these supposed actions of supererogation wouldn’t be good at all but would be condemnatory. The anti-supererogationist, then, should just bite the bullet with these examples, adding a caveat about deciding between competing obligations along the usual moral lines.

So, it seems that on one plausible reading of the gospels (and Article 14), there are no acts of supererogation in principle. Even if we were sinless, we still couldn’t exceed duty, we could only fulfill it perfectly – as, in some sense, God fulfills it perfectly. And given that the duty is so great that exceeding it isn’t even possible, how much more then will we be unprofitable servants?

I happen to think that supererogation will also be incoherent on many secular account of ethics. Morally relevant characteristics or goals (e.g. having the virtues, or maximizing good consequences) don’t really admit excesses. You can’t have more integrity than virtue demands, for example. On deontological pictures it’s not clear at all how to construe supererogation. Supererogation is supposed to be a morally relevant concept, but if morality is bound up with the notion of duty, how could moral demands be exceeded but still be morally praiseworthy? If you are interested in how smart people respond to my simplistic criticisms, read the Stanford Encyclopedia article, linked above.

Supererogation has an even lesser known sister concept: suberogation, or Richard Swinburne’s preferred terminology, “infravetatory” acts. These are acts which are in some sense bad to do, yet permissible. As you might imagine, this concept has an even harder time making sense than supererogation. The Stanford article briefly discusses this concept.