No matter whether one adopts a deontological or consequentialist account of ethics it is apparent that there exists a moral imperative to prevent genocide. To what extent and to what cost this imperative must motivate our actions is, I suppose, a subject of serious debate, however. But how can we define genocide? Surely we can all agree that the murder of 10,000,000 people constitutes genocide. But what if we subtract one fatality? Still genocide, of course. Minus one more? The same is still true. But at some point that logic fails; when we get down to the death of one, a few, or no people we certainly no longer have a case of genocide on our hands. It seems there is a sorites paradox here. If the number of people killed is ultimately arbitrary, how is the concept of genocide meaningful? Surely we can still find moral value in the deaths of millions (or even in the death of an individual), but it seems the label in itself is ultimately kind of subjective and meaningless.

The number of victims is not the only consideration entering into the judgments of whether a genocide is taking place. Other relevant factors are the nature and size of the victim group and the motivations and intentions of the perpetrators. Still, we can hold these other factors fixed and ask your question again, for example: hypothetically lowering the number of people killed, maimed, raped, and otherwise brutalized in the Rwanda genocide, when do we reach the point at which the genocide label would no longer be applicable? Or: at what time, in those horrible months of early 1994, did the daily decision of the world's leading governments not to intervene become a decision to ignore a genocide?

You're right that there is some vagueness here. But this does not render the term meaningless. As Wittgenstein writes, there may be some unclarity about where exactly the boundary lies between two countries -- say between China and Russia -- but this does not entail that it's unclear on which side Beijing or Moscow fall. Similarly, for many terms widely used in the criminal law, terms like "negligent," "reckless," "due diligence," "reasonable person," and so on. Even the word "kill," which you seem to find unobjectionable, is subject to a Sorites problem. Suppose you hurt someone and, as a result, she dies earlier than she would otherwise have done. Have you killed her? Surely yes, if she dies within seconds of your action. Presumably no, if she lives another 80 years rather than 81. So how long exactly must she survive for you to escape the killer label?

We confront and resolve such questions all the time in legislation and jurisprudence, and the borderlines are surely arbitrary to some extent. Here "genocide" is basically in the same boat with lots of other terms and, if we tossed all those terms overboard, we'd have very little language left.

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