" in learning how to imagine x, you gain abilities; later you have all the relevant imaginative abilities you had before, and more besides. and you notice, a priori, relationships of coherence or incoherence between attitudes that might figure in the realisation of x; later you are aware of all that you had noticed before, and more besides. and you think of new questions to explore in your imagining...and later you have in mind all the questions you had thought of before, and more besides. "

― David Kellogg Lewis , Philosophical Papers, Volume II