What part do truth and evidence play in our beliefs? A larger part in ours than in those of others, of course, for we are, or at least consider ourselves to be, more rational almost by definition than anyone else. But if we are honest, which we so rarely are, we must admit that we generally fall far short of Bertrand Russell’s notion of the rational man, which is to say he who holds his beliefs in exact proportion to the evidence in their favor. I have never met anyone like that, and Russell certainly wasn’t like that himself.
I suppose the nearest I ever came to changing my beliefs in pure accordance with the evidence is when I read the papers in The Lancet that proved that peptic ulceration was caused in the majority of cases by infection with the bacterium Helicobacter pylori. Until then, all sorts of theories of causation had been proposed, generally a mishmash of dietary, physiological, and psychological factors. Helicobacter swept them all away, like last year’s best-sellers, and I was converted without an inner struggle.
On reflection, the ease with which I was converted was attributable to the fact that I had previously held no other theory of causation of peptic ulceration of my own, strongly or even weakly. I had no belief at all to defend, and therefore was unusually open to the evidence. The mind abhors a vacuum, and Helicobacter encountered no difficulty in filling it.
This is not a typical situation, however. For example, I have just read a paper in The New England Journal of Medicine about a proposed genetic mechanism of obesity. Irrespective of the evidence presented, I admit that I was viscerally ill-disposed to an explanation that almost certainly will be used to absolve mankind from its own folly, from its increasing tendency to blubber, which is particularly grotesque in the Anglo-Saxon world but is spreading at an alarming rate elsewhere.
The reason I was hostile even before I had read the paper fully was its threat to my deeply held belief that obesity is not straightforwardly a disease like any other, but rather (in most cases) the consequence of human weakness. I hold this view not only or even mainly because of the evidence in its favor, but because I am afraid that to hold the opposite view, that obesity is in principle no different from, say, Parkinson’s disease, is to turn mankind from subject to object.