This comes down to occam's razor, basically.
Uniformity of process across time and space: If a past phenomenon can be understood as the result of a process now acting in time and space, do not invent an extinct or unknown cause as its explanation.
Though similar to the uniformity of law, this deals with geological causes, not physico-chemical laws. "We should try to explain the past by causes now in operation without inventing extra, fancy, or unknown causes, however plausible in logic, if available processes suffice." This is known as the scientific principle of parsimony or Occam's razor. "Strict uniformitarianism may often be a guarantee against pseudo-scientific phantasies and loose conjectures, but it makes one easily forget that the principle of uniformity is not a law, not a rule established after comparison of facts, but a methodological principle, preceding the observation of facts . . . It is the logical principle of parsimony of causes and of economy of scientific notions. By explaining past changes by analogy with present phenomena, a limit is set to conjecture, for there is only one way in which two things are equal, but there are an infinity of ways in which they could be supposed different." Gould simplified the issue, noting that Lyell's "uniformity of process" was also an assumption: "As such, it is another a priori methodological assumption shared by all scientists and not a statement about the empirical world."
So anything at all could be a reason something has happened. How do we discover what is most likely true?
Take stories of alien abduction. Are alien abductions really happening the way people say they are?
To quote Feynman:
"From my knowledge of the world that i see around me, I think that it is much more likely that the reports of flying saucers are the result of the known
irrational characteristics of terrestrial intelligence rather than the unknown rational efforts of extra-terrestrial intelligence."
Or in other words, what explanations do we have at hand that we can verify actually do occur as outlined in the explanation... in the real world? We should try to explain the things we see in terms of the regular old understanding we have at hand before we start trying to invent entirely new ones.
Far more often than not it turns out that strange phenomena can be explained with what we already know, if we are clever enough to see how it all fits together.
So what would explain the stories of alien abduction? The most obvious answer is that people are making it up.
Sure. It's POSSIBLE that (un observed) aliens are here probing our butts for some bizarre reasons, but it is far more likely that people are using their demonstrated imaginative minds to invent fictional stories, which people can be seen doing at all times, and in all places.
So it's also possible that light is doing entirely different things out there than it does in our own galaxy. Or solar system. Or atmosphere. Or the inside of our rooms. Maybe there is nothing beyond the door to this room? Just eternal blackness and the potential for existence if only i would open the door?
Or maybe light is doing what it always does... even when i'm not looking at it.