John Ioannidis is a very smart man who studies how we know things are true.
You know how sometimes grown-ups will say something is true, but you want to know how they actually know that? Well, John Ioannidis tries to figure that out for lots of things, like medicine or science.
He looks at how people study things to see if we can really trust their results. He asks questions like, "Did they test a lot of people or just a few?" and "Did they study the thing for a long time or just a little bit?"
He also thinks about how people might have made mistakes or been biased when they were doing the research. He tries to make sure we can really trust what we know, so we can make good decisions based on real facts instead of just guesses.