Verification and Falsification
The process of science is undertaken through two similar but distinct paths; verification and falsification. The two, though different, have more similarities than they have differences. Verification and falsification are based on two strands of knowing something; these are empirical data and rationality.
Empirical knowledge is basically that knowledge which is presented to our senses. Direct empirical knowledge is generally considered reliable and so is a route to knowledge. If I can report that there is a white thing in front of me that appears to have the characteristics of a wall, then it is reasonable to assume that I am standing in front of a wall.
Taking a step away from this direct knowledge does lead us away from certainty. For example, if I was to claim that yesterday I had a wall experience then I am adding another category of explanation to my wall experience, that of memory. A remembered experience is not as reliable as a current experience. But a current sensory experience is one of the best and most reliable chunks of knowledge that we can have.