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MARY WARNOCK (THES, October 31) describes the principle of double effect as "notoriously shifty".

I agree it can be shifty, but only when it is abused, which happens when the four criteria, which are very demanding, are not all fulfilled. The criteria are:

* the aim must be good

* the evil effect may be permitted but not desired

* the good effect must be better than the evil effect

* the action in itself must be morally licit.

So, for example, if a doctor gives a higher dose of pain-killer than is necessary to ease pain, knowing that that dose will shorten the patient's life, and knowing that a smaller dose would relieve the pain, then the principle of double effect is not fulfilled.

This principle, based on natural law - and therefore valid for all times and places - provides a clear guideline for judging the morality of an action.

It is disappointing that someone who is an ethics adviser does not stress the importance of the holistic approach so much sought after by the healthcare professionals looking after the dying.

Anne Dickinson

London, N1