If you'll recall, one answer is causal regularity:
Causal regularity: In order to have a science of, say, photons, photons must have some specific effects they produce by their very nature. If they had a different effect every time, then experiments could never reliably be extrapolated to every photon in the universe. When we do the double slit experiment on photons, we need to be able to say "this is what ALL photons would do under similar conditions." As a result, we can then have knowledge of the nature of photons.This means that something that has an effect must have that effect by its very nature. A few experiments on an object will reveal that nature, and its effect, which can then be extrapolated to all such objects of that type, thus making knowledge possible. We don't have to examine every single object of that type to know what it does.
If this principle were false, then an experiment involving one or a few photons could never be extrapolated to all photons, since each one might have a different effect.
This also entails that an object "points to" its effect (or range of effects), even if that effect is never in fact realized. So for example a match which is never struck nonetheless "points to" fire but not ice. Or a quantum vacuum "points to" the production of virtual particles but not elephants, even if it never actually produces any virtual particles. Or a volcano "points to" the production of magma but not tulips, even if it remains forever dormant.
Note also that these things may have other "accidental" effects. The volcano may cause bird migrations and mudflows, but these are side effects. Volcanoes by their very nature point to the venting of magma as their effect. If volcanoes did not do this then they just wouldn't be volcanoes. However, it could be the case that volcanoes never cause bird migrations or mudflows, but even so they would still be volcanoes. So some effects are mere side effects, but other effects are tied to the nature of the object in question.
There is a tight two-way correlation between cause and effect:
- A is the cause of B
- B is the effect of A
And a cause "points to" its effect:
- A "points to" B, even if B is never in fact realized