Here is how Lewis suggests we think of the a priori. The anticipation of Quine, it will be noted, could not be stronger. This is from Mind and the World Order, written in 1929, long before Lewis’ own students—Quine and Goodman (and Morton White) bizarrely started to use these very thoughts against Lewis himself. And it is very similar to Peirce’s view of how we should regard what we find indispensable. A priori “truths” are not necessarily true. They are simply what we need to articulate our world view. They could be revised, but only at great cost to that world view.