Conjecture, as in it has not been proven. Let's say it's a fairly obviously correct conjecture, such as twin prime conjecture or Goldbach's conjecture, though feel free to substitute your own. And yes, your algorithm will end up using numbers outside of the number-space in which the conjecture has been proven correct by exhaustion.

So, do you use the conjecture in your spiffy algorithm or do you err on the side of caution?