In everyday arguments we often meet with such phrases as ‘That's rational, it is mere common sense’ used in conjunction to approve of or back up some particular statement. The juxtaposition of these everyday locutions embodies a profound truth, the truth, namely, that the basis of rational communication between human beings is plain common sense. I call this point profound because it has been missed in all the discussions about rationality and its basis that I know; certainly its elusiveness thus (...) seems to indicate that participants in these discussions have not delved deeply enough. But I concede that this truth is simple and obvious, and conclude, therefore, that it has been overlooked only because its very obviousness has been taken, wrongly, to indicate superficiality and inadequacy. In suggesting that it is neither superficial nor inadequate I shall be relying on Wittgenstein's interpretation of Moore's ‘Defence of Common Sense’, to which I shall be adding a particular twist of my own. (shrink)