A philosophical problem has the form: I don’t know my way about. (Ludwig Wittgenstein)

Up until the late 19th century every observation was compatible with Newton’s theory of gravity. All these observations are also compatible with Einstein’s General Theory of Relativity. Two quite different theories were compatible with the same set of observations; therefore, one cannot know they have derived true theories from observations.

Assume we have a long series of numbers. They go on: 2, 4, 8 … What is the next number in the series?