I have acknowledged in relevant contexts in the body of the book a number of similarities between aspects of my thesis and ideas which have already been put forward. There are some further such
similarities which I have come across and which I have not yet mentioned because they happened not to be relevant. I wish here tentatively to acknowledge these also. I say ‘tentatively* because it seems possible that the philosophers concerned would deny any great significance to the similarities and would indeed object not only to the suggestion that we need a contextual criterion when testing empirical statements but also and a fortiori to the very general conclusions I have drawn.