As Popper put it.
One commentator objected strongly to this notion in a previous post, so I'm going to give some examples of how important this principle is. In everyday life I think it's true, but we can "bracket" it (i.e., set the problem aside). But in science we do so at great risk. Here are a few "plain" facts as seen by late-nineteenth century science:
1) Motion occurs over a continuum.
2) Energy increases or decreases smoothly with no discontinuities.
3) Objects do not gain or lose mass except that the mass comes from or goes to somewhere else.
4) Measurements of length are stable -- a meter is a meter is a meter, in whatever circumstances we do the measurement.
Well, folks, every single one of these "facts" turned out to be false. That is, they turned out to be theories.