Full Idea
It seems natural to claim that light rays moving in straight lines is contingent but a priori. Scientists stipulate that they are the standard by which we measure straightness, but their appropriateness for this task is a contingent feature of the world.
Gist of Idea
Light in straight lines is contingent a priori; stipulated as straight, because they happen to be so
Source
Edwin D. Mares (A Priori [2011], 02.9)
Book Reference
Mares,Edwin: 'A Priori' [Acumen 2011], p.31
A Reaction
This resembles the metre rule in Paris. It is contingent that something is a certain way, so we make being that way a conventional truth, which can therefore be known via the convention, rather than via the contingent fact.