There’s little that is subjective about shitting. Which is why shitting works as an example for Foot’s later work. She came to think that moral evaluation is not grounded in attitudes, preferences, decisions, or prescriptions, but in objective facts about the life-form of human beings.
In common with others, I took it for granted at that time that a discussion of the rationality of moral action would start from some theory or other about what a reason for action must be: rather favouring a desire-fulfilment theory, with some special allowance for the force of considerations of self-interest. I now believe that both the self-interest theory of rationality and the theory of rationality as desire fulfilment are mistaken. Moreover, there seems to be a mistake of strategy involved in trying to fit the rationality of moral action into either theory: such an enterprise implying that we first come to a theory of rational action, and then try as best we can to slot in the rationality of acts of justice and charity.
Natural Goodness, p. 10
The discussion to take a shit is not the result of ratiocination.
The following is the summative paragraph of the first chapter of Natural Goodness. She sets out her thesis clearly and briefly. I have bolded the most relevant sentence.
What, then, is to be said about the relation between‘fact’ and‘value’? The thesis of this chapter is that the grounding of a moral argument is ultimately in facts about human life—facts of the kind that Anscombe mentioned in talking about the good that hangs on the institution of promising, and of the kind that I spoke of in saying why it was a part of rationality for human beings to take special care each for his or her own future. In my view, therefore, a moral evaluation does not stand over against the statement of a matter of fact, but rather has to do with facts about a particular subject matter, as do evaluations of such things as sight and hearing in animals, and other aspects of their behaviour. Nobody would, I think, take it as other than a plain matter of fact that there is something wrong with the hearing of a gull that cannot distinguish the cry of its own chick, as with the sight of an owl that cannot see in the dark. Similarly, it is obvious that there are objective, factual evaluations of such things as human sight, hearing, memory, and concentration, based on the life form of our own species. Why, then, does it seem so monstrous a suggestion that the evaluationof the human will should be determined by facts about the nature of human beings and the life of our own species? Undoubtedly the resistance has something to do with the thought that the goodness of good action has a special relation to choice. But as I have tried to show, this special relation is not what non-cognitivists think it is, but rather lies in the fact that moral action is rational action, and in the fact that human beings are creatures with the power to recognize reasons for action and to act on them. This in no way precludes recognition of the part played by ‘sentiments’ such as (negatively) shame and revulsion or (positively) sympathy, self-respect, and pride in motivating human virtue. I think that David Wiggins is right often to have stressed this side of Hume’s moral philosophy
Foot rejects fulfilling desires and self interest as the place to start in considering our actions, in preference for objective facts about the human life.
Shitting shows us that the reason for an act can be entirely independent of our attitudes and desires.