In the Introduction to his 1965 Linacre Lecture on the nature of perception, Lord Brain1 (18951966) affirmed his conviction that philosophers have much to learn from physicians. This belief that medicine has knowledge which can be valuable to philosophy may seem novel to both philosopher and physician. In their interrelationship, the former always appeared to be the donor, the latter but a humble recipient. Philosophical value judgments guide the doctor in his decision on problems such as euthanasia, organ transplantation, abortion, contraception, and professional secrecy. What, if anything, can medicine offer in return?
It can offer a great deal, according to Brain. And, what is surprising, its contribution lies not, as might be expected, in the low ground of homespun reflection, but in the rarefied strata of metaphysical and epistemological speculation. In the Linacre Lecture, as well as in his other communications, Brain highlighted an important contiguous facet of