Rationality is a process, but it has to have data to work with. That data comes from perception of the world (sight, hearing, etc.). There is no such thing as "

Rationality is a process, but it has to have data to work with. That data comes from perception of the world (sight, hearing, etc.). There is no such thing as "pure" rationality without sense data to work with. Evidence for the big bang ultimately comes from sense data. You have to integrate a lot of sense data into scientific theories, which then are integrated with other scientific theories, and so on. Science has gotten so far - but it all started out with simple experiments, eventually microscopes and weak telescopes, etc. and built from there. Critical thinking allows you to overcome cognitive biases. Faulty perception is pretty easy to detect. A stick in a glass of water will look bent, but it's easy to figure out that it actually isn't bent.