 Second, Mill envisions that the harm principle is something that we can apply prospectively to prevent someone from acting in certain ways and causing harm. In many cases all we could reasonably know is that a given action risks harm. Fortunately, this seems to be all that Mill requires (IV 10). There are interesting and important questions about what threshold of risk must be met for purposes of the harm principle, which Mill does not address. Presumably, the threshold should vary inversely with the magnitude of the harm risked, so that the probability of harm required to justify regulation is lower the greater the harm risked.