The following quotation comes at second or third hand. John Alexander Smith (1863-1939), Waynflete Professor of Moral and Metaphysical Philosophy at Oxford, gave a lecture sometime before WWI, attended by Harold Macmillan. Macmillan reported Smith's words to Isaiah Berlin, and Isaiah Berlin told them to Ramin Jahanbegloo, who reproduced them in Conversations with Isaiah Berlin
(London: Phoenix Press, 1993), p. 29:
All of you, gentlemen, will have different careers -- some of you will be lawyers, some of you will be soldiers, some will be doctors or engineers, some will be government servants, some will be landowners or politicians. Let me tell you at once that nothing I say during these lectures will be of the slightest use to you in any of the fields in which you will attempt to exercise your skills. But one thing I can promise you: if you continue with this course of lectures to the end, you will always be able to know when men are talking rot.