Elizabeth Anscombe, answered from the texts and cited to the page.
There is a real moral difference between consequences that the agent intends — as ends or as means — and consequences that the agent merely foresees as side-effects of what he intends.12 The Principle of Side-Effects (PSE), as I name what others call the Doctrine of Double Effect, makes this difference morally load-bearing: some things, if aimed at as ends or as means, necessarily render the action a bad action; the same things, if brought about as foreseen side-effects of an action, do not necessarily render that action bad.3
Consequentialism — the doctrine I named in MMP and traced from Sidgwick onwards — denies this difference. Once the difference is denied, everything follows: there are no longer absolute prohibitions (since every prohibition can be overcome by sufficient consequences); responsibility inflates beyond practicability ("my business is the human race"); and the moral character of what an agent does collapses into the numerical aggregation of what happens.
Equally, the lax abuse of double effect by some Catholic casuists — directing intention away from a foreseen effect that is in fact the means — corrupts the doctrine in the opposite direction. PSE is meant to do work, not to license sophistry.4
This is not a meaningless stance. They'd be shewing themselves as people who will reject any policy making the death of innocent people a means or an end. But, to repeat: you cannot deduce the permissibility of moving the rock from the principle of side-effects. It determines nothing about that, except perhaps that it is not excluded on the score of My references are to his Ambiguity in Moral Choice.Human Life, Action and Ethics, [3]
By Sidgwick's doctrine, there is no difference in his responsibility for ceasing to maintain the child, between the case where he does it for its own sake or as a means to some other purpose, and when it happens as a foreseen and unavoidable consequence of his going to prison rather than do something disgraceful.Human Life, Action and Ethics, [5]
The Principle of Double Effect Catholics, however, can hardly avoid paying at least lip-service to that law. So we must ask: how is it that there has been so comparatively little conscience exercised on the subject among them? The answer is: double-think about double effect.Part One: Ethics, pp. 69–70
A divine law theory of ethics is reduced to an insignificant variety by a footnote telling us that 'the best theologians' (God knows whom he meant) tell us that God is to be obeyed in his capacity of a moral being.Human Life, Action and Ethics, 1178b16
MMP coined the term "consequentialism" and traced it from Sidgwick through the post-Sidgwick English tradition (Moore, Ross, Prichard, the early-twentieth-century Oxford moral philosophers) to the post-war utilitarian and proto-utilitarian writers Smart represents. Diamond's "Consequentialism in Modern Moral Philosophy" (1997) defends the classification. The Doctrine of Double Effect / PSE is defended in modern form by Quinn, Kamm, and the Catholic moral tradition; engaged sympathetically by Foot from the 1967 trolley paper onwards; rejected by Smart, Singer, and the consequentialist mainstream; empirically investigated by Joshua Greene and the moral- psychology tradition.