Science assumes it must discover its knowledge. Science presupposes a regular order to nature and assumes there are underlying principles according to which natural phenomena work. It assumes that these principles or laws are relatively constant. But it does not assume that it can know a priori what these principles are or what the actual order of any set of empirical phenomena is. A scientific theory is a unified set of principles, knowledge, and methods for explaining the behavior of some specified range of empirical phenomena.
Scientific theories attempt to understand the world of observation and sense experience.
They attempt to explain how the natural world works. A scientific theory must have some logical consequences we can test against empirical facts by making predictions based on the theory. It is true that some scientific theories, when they are first developed and proposed, are often little more than guesses based on limited information. On the other hand, mature and well developed scientific theories systematically organize knowledge and allow us to explain and predict wide ranges of empirical events. In either case, however, one characteristic must be present for the theory to be scientific.