Hume's principle

Hume's principle is an idea by a Scottish philosopher named David Hume. He said that it is impossible to be certain of something unless it is based on facts or demonstrated by experience. He used the example of a square circle, an imaginary shape which has the properties of both a square and a circle but doesn't exist in reality. According to Hume, it is impossible to have certainty this shape exists because it cannot be observed in the real world. Hume's principle states that everything we think we know must be supported by facts or evidence, otherwise it cannot be accepted as true.