What was the greatest discovery in human history? Was it the wheel, the nail, the printing press? Was it fire? the internet? Not one of these, nor anyone of 100 others.
The greatest discovery in human history — one that is prior to every other discovery — is that our traditional sources of belief are in fact generators of error and should be dismissed as grounds for knowledge. These include faith, revelation, dogma, authority, charisma, augury, prophesy, intuition, clairvoyance, conventional wisdom, and subjective certainty.
How, then, can we know? Other than by proving mathematical theorems, which are not about the material world, the answer is the process that the philosopher Karl Popper called conjecture and refutation.
We come up with ideas about the nature of reality, and test them against that reality, allowing the world to falsify the mistaken ones. The “conjecture” part of this formula, of course, depends upon the exercise of free speech. We offer these conjectures without any prior assurance they are correct. It is only by bruiting ideas and seeing which ones withstand attempts to refute them that we acquire knowledge.
Steven Pinker, Why free speech is fundamental
The Boston Globe, January 27, 2015