It is undesirable to believe in a proposition when there is no ground whatever for supposing it true
Bertrand Russell's idea, put forward 80 years ago, is about as dangerous as they come. I don't think I can better it: "I wish to propose for the reader's favourable consideration a doctrine which may, I fear, appear wildly paradoxical and subversive. The doctrine in question is this: that it is undesirable to believe in a proposition when there is no ground whatever for supposing it true." (The opening lines of his Sceptical essays).