Science is a process that builds upon existing theories and knowledge by continuously revising them. Every aspect of scientific knowledge can be questioned, including the general rules of thinking that appear to be most certain. So why is science trustworthy if it is always changing? If tomorrow we will no longer see the world as Newton or Einstein found it to be, why should we take seriously today's scientific description of the world?
The answer is simple: Because at any given moment of our history, this description of the world is the best we have. The fact that it can be made better can't diminish（降低） the fact that it is a useful instrument for understanding the world.
Consider a folk healer's herbal medicine. Can we say this treatment is "scientific"? Yes, if it is proven to be effective, even if we have no idea why it works. In fact, quite a few common medications used today have their origin in folk treatments, and we are still not sure how they work. This does not imply that folk treatments are generally effective. To the contrary, many of them are not. What distinguishes scientific medicine is the readiness to seriously test a treatment and to be ready to change our minds if something is shown not to work. A research doctor in a modern hospital must be ready to change his theory if a more effective way of understanding illness, or treating it, becomes available.
What makes modern science uniquely powerful is its refusal to believe that it already possesses ultimate truth. The reliability of science is based not on certainty but on a complete absence of certainty. As John Stuart Mill wrote in "On Liberty" in 1859, "The beliefs which we have most warrant （依据） for, have no safeguard to rest on, but a standing invitation to the whole world to prove them unfounded."
