The present chapter borrows its title from a paper by Michael Polanyi that has already been published three times (in English), and which, no doubt, will deservedly be published more. The present discussion has, indeed, developed out of comments on that paper by Polanyi — both were read in the Boston Colloquium for the Philosophy of Science, 7 April 1970. Yet it is not necessary to add the subtitle "Comments on Polanyi's paper' because it might just as well read, "Comments on the whole literature on the topic'.