The opinions people commonly hold including their religious, philosophical or ethical beliefs are often not very well grounded. But in science questions are constantly being raised regarding the bases of specific assumptions. How does the scientist reach the assumptions she uses as explanations, and how can she be sure that those hypotheses are true? For if our belief that they are true is to be warranted, we require that scientific explanations be justified. And the demand made on scientific assumptions is not simply that they be justified. The justification has also to be reliable. This is where scientific method plays its crucial role. It must ensure that justifications are reliable. So when science provides us with explanations, they are grounded in the evidence available to the researcher, and the method ensures that the evidence is such as to make those explanations well-justified beliefs. The method lays down the rules for discovery and justification and makes scientific practice rational and intelligible.