 Empirical belief and a priori belief aren’t logically separated but intertwined for Lewis. The empirical belief that there are no unicorns presupposes the concepts of negation and unicorn, and thus more general a priori principles governing negation and explicating the concept of unicorn, e.g. unicorns are horse-like creatures with a horn in their nose. Repeatedly failing to apply the concept of unicorn successfully to experience may make it extremely likely that there are no unicorns and eventually lead us to drop the concept altogether from our conceptual repertoire as useless clutter, along with beliefs explicating the concept, but it doesn’t do so by disconfirming or making any less likely the belief that if anything is a unicorn, it is a horned horse-like creature. More important cases Lewis discusses are ones where we discover that there are no Euclidean figures in our space and cease employing Euclidean geometry to interpret experience, or ones where cruder categories for interpreting experience are replaced by more fine grained ones that carve up experience in novel ways that are more valuable for our purposes, or ones where inventions open up the bounds of experience and lead us to abandon an old theory that can accommodate such experience but in a less simple way than a novel one. Categories for Lewis don’t really change or alter but are given up and replaced, and old a priori truths are replaced by new ones, not contradicted by them and not made false by them (Lewis 1929, 267–8). 