According to Duncan Richter in his book, Anscombe's Moral Philosophy, her position on what is good behavior boils down to this: There are certain states of affairs to do with human beings (being a human) which are part of the nature of humankind. That is, they represent practices, institutions, ways of being in the world that are part of what it means to be human. Those acts that are morally good will be seen to express, reflect or be consistent with those human activities and institutions. Given her strong religiosity (she was a committed, practicing Catholic) she holds that these natural human propensities are part of God's plan for mankind and so consistent with what God wants for us. Therefore, to abide by them is to conform with God's will. But we do not do them because God commands it. Moral actions (or ethical behavior, since she rejects the notion of "moral" as inappropriate for our age -- a period in which we view the divine differently than mankind once viewed it) is thus to be consistent with God's will but not necessarily to act out of the desire to obey.
Anscombe thus rejects a morality based on duties. To have duties implies some kind of adverse repercussions for failing to obey them but moral claims (perhaps "ethical" claims better captures her view here) have no such consequences in and of themselves. That is why we can act immorally if we like, with impunity (other than some prospect of a judgment visited upon us in the afterlife which not everyone will grant even though everyone is presumed morally assessable in this life). Duties imply commands and an enforcer who punishes deviance from those commands. To suppose one has duties without the possibility that one can suffer adverse consequences for specifically failing to fulfill those duties is to render the very notion of duty empty. Richter doesn't maintain that Anscombe rejects the idea of duty per se though. He only suggests that she rejects it as a basis for behaving ethically. Instead she wants to say that we behave in the ethically right way because we come to recognize what is good for us as human beings.
Now this doesn't mean that what is good for us in that sense, i.e., what is good for humanity in general, is good for us specifically. At any given point in our assessment about possible courses of action we may determine that something that is good for humanity in general (promise keeping, fidelity, loyalty to those deserving of it, charity, etc.) may not get us the most benefit. In fact, it may actually do us some harm. But, on Anscombe's view, a sufficiently enlightened person will see that the important thing is to act in a way that is good for humanity in general, according to the nature of what we humans are, rather than in a way that may benefit us individually but be harmful to humans in general. Presumably this reflects what she appears to take as the plain fact that, being human ourselves, we also and always have a stake in what is good for humankind. To the extent humans in general are served, so are we (even where we may not be well served individually by the action or omission). The wise or enlightened person will presumably see how the good of humankind is good in a way that transcends what is good for him or her as an individual. This is akin to the old Aristotelian notion of eudaimonia (which Richter and others translate as human flourishing) -- that what is good in an ethical sense is that insofar as it reflects or represents what is the ideal state or condition in which human beings should be. Yet Anscombe gives no affirmative reason as to why any of us should care about humanity in general and seems to rely on the general notion that being part of humankind is enough.
God's role in this, for her, is as the founder or author of this system, of what is and what we understand. Our role is in the practice of coming to understand. Part of coming to understand is, for Anscombe at least, to see God's role in this. But it appears that it is not essential because Anscombe seems to think, on Richter's view, that to recognize that there are certain moral (oops, "ethical") imperatives, which are part and parcel of being human, is a strong enough reason to be good. The problem, though, must surely be what do we do, what can we do, when someone fails, or claims to fail, to see the things that we see as inherently good for humans, as humans, to do? And what do we tell the individual who acknowledges seeing such things (or things like them) but maintains indifference to such concerns nonetheless?
Can this Anscombian view provide a genuine basis for making moral claims which, at the least, imply the possibility of arguing for or against them? While it always remains possible to argue about particular cases -- what the facts are -- the real problem in making and holding moral claims, it seems to me, must be to determine what actions have value for us and why (which last implies the ability to make the case for our value judgments). Once we remove from the equation disputes over fact, including disputes over which actions are more likely to have what results, etc., the value question of why we should do one thing instead of another remains. That is, we are still obliged to show how we can derive an ought-to-do from a this-is-how-it-is claim (ought from is).
Anscombe's view suggests that this why question is answered by coming to more fully understand what we are as human beings. But can't we understand that and still reject it? Does recognizing that humans are inherently social creatures, and that being social requires participation in certain practices and institutions (which sustain a social way of being) and rejection of others, oblige us to choose to participate and reject in the same way others do? Why should we value any particular act or way of acting if it doesn't serve our own particular interests? Why should we care what is good for humanity in general if it's not good for ourselves?
Does this Anscombian approach to explaining and justifying moral behavior by recourse to a theory of natural goods, discernible by human intellect, actually solve the moral conundrum left us by G.E. Moore (intuitionism -- a claim that "good" is an intuitively known "non-natural quality"), the logical positivists, like A. J. Ayer (emotivism -- value claims serve only to express how we feel), or their successors like R. M. Hare (prescriptivism -- moral language is about directing behavior, not describing it). Don't all these approaches finally amount to a kind of moral subjectivism which fundamentally undermines the moral project because it removes the ability to find and argue for rational supports for moral claims?
Does Anscombe's naturalistic (and religiously conformist) approach offer an effective antidote to the moral valuing problem kicked up by these earlier thinkers?