There is a constant temptation in philosophy to postulate a realm of phenomena in which nothing is hidden from us. Rene Descartes thought that one's own mind is such a realm. Ludwig Wittgenstein' enlarged the realm to everything that is of interest to philosophy. That they explained this special feature in very different ways hardly needs to be said; what is remarkable is their agreement on our possession of a cognitive home in which everything lies open to our view. Much of our thinking – for example, in the physical sciences – must operate outside this home, in alien circumstances. The claim is that not all our thinking could be like that.