Two papers I love: “Morality as a system of hypothetical imperatives” and “Moral beliefs.”
In “Moral Beliefs” she argues, among other things, that there are conceptual limits on the use of moral vocabulary, that is, moral terms have some sort of fixed content, rather than being terms for the expression of noncognitive states akin to boo or hurray. In her famous example– forgive me, my memory of this is unclear and the book is packed away– she says that a man clasping his hands three times in an hour simply cannot be splendid or brave or whatever unless there’s some story tying those actions to some recognizable good (e.g. if he’s signaling in code or overcoming a stroke or something).
This paper is in opposition to views like RM Hare’s, which say that moral judgments are such because of their formal features, e.g. they endorse or command; they have no fixed content.
“Morality as a system of hypothetical imperatives” (a paper she later partly recanted) argued that there are two senses in which moral judgments might be said to be “categorical.” First, we apply them to people without regard for their motivations or desires. That is, we don’t take someone’s lack of desire to tell the truth to render “you ought not to lie” inapplicable or incorrect. Second, (true) moral claims guarantee reason for complying. That is, if it is true of you that you ought to F, then you have reason to F. Foot argues that moral claims are categorical in the first sense but not the second. That is, we apply them without regard for motivation or desire, but they are not thereby reason-giving independent of motivation or desire. Her famous example involves club rules or rules of etiquette: a failure to reply in the third person to an invitation addressed in the third person is rude whether or not you care about avoiding rudeness. But this by itself does not show that you have reason to avoid rudeness. So too with morality.
She wrote a lot of great stuff– these are just personal favorites. And she gave us the trolley problem!