Occam's Razor (A.K.A. "Okham's Razor")
You may have noticed that people faced with unpleasing evidence often introduce assertions about external evidence to distract the ordinary logical process that might lead one to causes or effects of the evidence. If the evidence is something we all can share by ordinary observation (winter follows summer, the force of gravity acts upon objects with an accelerating force of 32-feet-per-second-per-second), then we need not worry. However, when explanations we are offered require acceptance ("a priori" or from the beginning without adequate demonstration of its existence) of intermediary entities, like demons or angels or aether or phlogiston, William of Occam (1285-1381) argued that we ought to eliminate those explanations in favor of those which require fewer invisible and indemonstrable entities to do their job. Phil Gibbs (and Sugihara Hiroshi) restate the principle in readily understandable language:
"Entities should not be multiplied unnecessarily."
"When you have two competing theories which make exactly the same predictions, the one that is simpler is the better."
If you doubt the importance of the "Razor," consider the number of invisible complicating entities your parent or sibling could introduce to explain why they're right, if not checked by it as a result of its general cultural acceptance. This was a principle that won its place in human thought by a long hard road.