“Crying is all right in its way while it lasts. But you have to stop sooner or later, and then you still have to decide what to do.”
—C.S. Lewis, The Silver Chair (1953)

We hear it all the time: “One death is one too many!”; “One rape is one too many!”; “One <insert moral outrage here> is one too many!” This rallying cry is used to justify and demand any sacrifice in the cause of preventing the bad thing. The logic is straightforward: if one x is one too many, then the only value of x that can be permitted is 0.

But this is to misunderstand the nature of decisions. A decision is a trade-off, in which different factors are weighed up against each other. Even when the resolution of a decision seems obvious, the trade-off has been made and concluded.¹

Analysis of Terms

The “One Is One Too Many” (OIOTM) class of statements function as categorical moral imperatives. A syntactically related group of statements take the same form, but are hypothetical imperatives in meaning, even when not in form, for example: "One alcoholic drink is one too many … in order to optimise your health". They are intentionally referring to only one part of the trade-off, emphasising that element of it while not denying the existence of other values; in this case, such another value might be “…to maximise one's pleasure”. But the full OIOTM moral statements are instead absolute, not hypothetical: they lack a “because…” clause. At most, they are contingent on morality itself, but this is the same as being absolute: "You should do x because you should" is the same as there being no "because…" clause at all.

These OIOTM statements are also different from another group of syntatically similar statements, such as: "Two deaths are one too many". The latter type of statement tends to occur when there has been some sort of rational trade-off made of the costs and benefits of preventing, say, a given number of deaths. “We have assessed the costs and benefits, and concluded that x deaths would be the marginal death that started minimising marginal utility. (It follows of course that in some cases, a very low number of deaths are too few deaths! This means that the cost of the precautions that have led to so few deaths has been too great: the trade-off was imbalanced.) It is a numerical quantifier, i.e. a form of the existential quantifier—but OIOTM is a universal quantifier.

This is very similar to Russell's quantification of the definite article. “One death is one too many” can be formalised as