but it's not a fact until he investigates it enough to prove it.
The sciences don't deal with proof precisely because there is no answer to the question "when is something investigated enough?". My 2 default examples are from physics and the cognitive sciences. What we call classical physics was (and in many ways still is) the most spectacularly successful scientific framework ever. It is the epitome of the so-called "hard sciences". Then, early in the 20th century, something happened. About a hundred years before Einstein's work in 1905, Young had successfully demonstrated ("proved", for our purposes) that light was a wave. It was a relatively simple experiment that you can reproduce for yourself. There is no way for particles to exhibit interference effects. Young showed light did, and thus had to be a wave. Then, early in the 20th century Einstein showed it was composed of discrete "packets" or particles. Neither researcher did anything wrong. Both experiments were sound, but they contradicted one another. There was no way for them to be both correct and nothing wrong with either, so what was the problem? The entire framework of classical physics. Neither particles nor waves really exist and pretty much all of physics up to that point turned out to be wrong (often approximating accuracy, but wrong nonetheless).
All experiments are performed within a framework. For example, for the last 30 odd years two general theories of cognition have existed in contradiction to one another. Embodied cognition holds that even fundamentally abstract reasoning relies on sensorimotor programs. So, for example, these is no language faculty or language module but rather language is a domain general skill that relies on things like metaphorical extensions of our physical experience (embodiment) in the world. There are decades of experiments behind this view, yet it is rejected by classical cognitive scientists. Why? Because they have different starting assumptions and therefore interpret experimental results differently. I once asked the head of my lab what would happen if embodied cognition scientists finally performed good enough experiments such that there were no alternate explanations possible. He said that we'd have to suspect the tools used were the problem. Why? Because he believes the framework itself to be so supported that if we don't find the evidence we should, we aren't looking the way we should.
It's a good thing that quantum physics didn't require any very complicated experiments, because nobody wanted it and nobody liked it. It was antithetical to the entire scientific program up to that point. However, there was no way around it. We weren't dealing with massively complex systems, incredibly vast data sets (as in e.g., an fMRI scan), lots of qualitative variables, etc. We were dealing with what was quite literally at that time the simplest systems imaginable. Yet we still had contradicting results. And now the three most successful physics frameworks, classical mechanics, relativistic physics and quantum mechanics are likewise in conflict with one another.
