In the Michaelmas Term 1968 I gave a course of lectures on the Philosophical Investigations. Until then nobody had lectured at Cambridge specifically on that book, though it had been in print for fifteen years and must by that time have been lectured on in nearly every other philosophy department in the English-speaking world. One reason why we were so slow is suggested by a remark that John Wisdom made after hearing Max Black give a lecture on the Tractatus in the early fifties. As we came out of the lecture room he said to me ‘That was a strange experience. I have a clear memory of all that from my early years in Cambridge. And yet in some ways it was like hearing a lecture on Spinoza.’