All scientific propositions must be testable and potentially disproveable. Otherwise, those propositions are merely philosophic and not scientific.
So science is precarious, built on relative, probable truths that can be negated by a single counter-example. Historically, this has been known as the problem of induction, but more recently, a man named Nassim Taleb has re-awakened our skepticism with several engaging books, the most successful of which is entitled, The Black Swan.
In that tome, and his related works, he makes a compelling case for doubting what we know and what is knowable. He goes on to apply these insights into varied aspects of modern life, including epidemiology and high-finance with policy prescriptions for individuals and governments alike.