The distinction between what is intended and what is not intended but brought about as a side-effect is at the basis of the vast modern law of tortious liability in negligence; it is the focus, too, of the criminal law's long-accepted distinction between murder and manslaughter. As those facts suggest, it is not the esoteric preserve of some sectarian moral teaching, but a morally significant distinction which is intrinsic to practical reasonableness.

But accounts of it, judicial, legal-academic, and philosophical, remain deeply confused. The confusion can be traced to two main sources: (i) failure to distinguish free choice from spontaneity, and rational from subrational motivation, and (ii) lack of an ethical theory clear enough to identify the variety of different moral norms which human conduct is required to satisfy, some norms bearing only on what one intends (and does) but others bearing, in other ways, on what one foresees and thus somehow controls as the side-effect(s) of what one intends (and does).

It would be good to show how several philosophical accounts of modern tort law – notably those inspired by economic analysis – overlook the significance of the intention/side-effect distinction for the entire structure of tort. But in this essay I shall attend only to criminal law. In that restricted context, I shall try to advance a clarification of issues by discussing a newly, though not wholly, clarified judicial understanding of intention and side-effect.