The impulse to philosophy is often provided by leprechauns, mischievous little sprites who lurk at the end of fine chains of reasoning and make trouble. They delight in absurdity and paradox, and are especially happy to help ambitious thinkers dig their own graves. Philosophers spend much of their lives trying to put a stop to leprechauns or fencing them out with the aid of reason. But in truth the relationship is symbiotic. If a realm of thought can be made fully coherent, there is no work for philosophers to do in it. Science is the obvious case but the remark applies, paradoxically, to philosophy itself. A philosopher who solved all the problems of philosophy would be about as popular as a huntsman who shot the fox or a theologian who proved the existence of God. Luckily there is no serious risk. At least on the view of philosophy to be taken here, there will always be leprechauns to keep us busy.

The view is that philosophy flourishes on the border between closed and open questions. By a closed question I mean one where the method of answering is not in doubt. Science cannot as yet fully identify or isolate the AIDS virus and cannot immunize against it. But, provided that no revolution in medical techniques is required, ignorance does not stop questions about it being closed. On the other hand there is some reason to suggest that AIDS research needs to include psychological and sociological understanding and what counts as a revolution in technique is unclear.