Morality as a System of Categorical Modals
We express our moral beliefs in sentences like these: “Murder is wrong”, “Stealing is wrong”, “Generosity is good”, and “Violence is bad”. What do they mean? Some have said they are equivalent to categorical imperatives, others suggest that hypothetical imperatives provide a better analysis; the concepts of duty or obligation are frequently brought in; the word “ought” is deployed. I want to suggest that the propositions in question are modal in character: the concepts of necessity and possibility can be used to analyze what moral statements mean. Thus “Murder is wrong” means “You must not murder”, and “Generosity is good” means “You must be generous”. The “must” here is specifically moral—it isn’t the “must” of logical necessity or metaphysical necessity or epistemic necessity. It isn’t that people can’t do otherwise than be generous, or that it is certain that people are generous. No, the thought is that one must morally be generous and not murder or steal. That is, there is a specifically moral “must” in our language (or our use of language). What is the evidence for this? We say things like, “You must visit your grandmother soon” or “I really must remember her birthday this year” or “We must do something about global warming”. We subscribe to such assertions as that promises must be kept, lies must not be told, gratitude must be shown, kindness must be practiced, justice must prevail. Some things we can do and not invite moral rebuke, but other things we can’t do without inviting moral censure. There is a contrast in our thinking between what it is possible to do morally and what it is necessary to do morally. These are perfectly good uses of “possible” and “necessary”, and they don’t mean what they do in non-moral contexts (modals are notoriously protean). They are entirely natural ways of expressing our moral attitudes. Thus moral language is modal language.
If so, we would expect modal logic to apply to moral language. One central rule of modal logic is that necessity implies possibility: what is necessary is possible. This rule has its counterpart in the well-known principle that “ought” implies “can”: you cannot be obliged to do what you can’t in fact do. If you must see your grandmother tomorrow, than it must be possible for you to see her. No one can say “I must cure cancer tomorrow” when everyone knows that this is not possible. As necessity implies possibility, so moral obligation implies the ability to carry out the action in question. Of course, “can” doesn’t imply “ought”, as “possible” doesn’t imply “necessary”. Thus the basic rules of modal logic apply to moral reasoning.
The modal analysis also comports with the sense we have that morality is binding—that it leaves us with no alternative but to do what is right. Morality restricts freedom; it cuts down our options. But so does modality generally: the number 2 has no option but to be even (it isn’t free to be odd), and epistemic necessity eliminates all alternative possibilities (I can’t not be in pain given my current epistemic situation). If I must repay that debt, then I have no alternative but to repay it—no moral alternative. I have no choice in the matter; I am not morally free to act otherwise. We are bound to do what morality requires of us, and moral necessity captures this idea.
Modality also captures the meaning of prudential statements, as in “I must eat less sugary food” or “I must exercise more frequently”. Again, this is a perfectly natural way to express our prudential attitudes. But the connection between morality and prudence is conceptually close: we can even view morality as a kind of extension of prudence to other people. If it is prudent for me to take care of my health, then it is moral of me to be concerned about other people’s health. The Golden Rule makes this connection explicit: do to others what you would want done to you, i.e. what you would do for yourself if you could. Thus I must do for others morally what I must do for myself prudentially—not harm them and promote their welfare. Given that a modal analysis of statements of prudence is plausible, so is a modal analysis of moral statements plausible. The same conceptual apparatus applies to both, i.e. modal concepts.
None of this is yet to say how these uses of “must” should be analyzed, and that will be open to debate depending on one’s view of the content of morality. I have only given a theory of the logical form of moral statements, taking the moral modals as primitive for this purpose. The canonical form of a moral proposition is, “Everyone must X”, where “X” ranges over action types (e.g. “Everyone must refrain from murder”). Moral sentences are a special type of modal sentence: that is their correct semantic analysis. If we were giving a truth-theory for moral sentences, we would first translate them into the corresponding modal paraphrase and then offer a Tarski-type theory of truth for these modal sentences (or some other type of formal semantics, according to preference). We might try to construct a Kripke-style modal semantics for moral discourse following these informal ideas.