Theories are like apples; there are good ones and there are bad ones. A good apple is flavorful and nutritious. A good theory is one that we have good reason to believe is true, or at least likely to be true. Apples have all sorts of features that are indicative of goodness and that can be used at the store for making smart choices. An apple's color, shininess, shape, and firmness are related to its flavor and food value. Similarly, theories have features that are indicative of their truth, and the task of justification is to identify these features and use them to guide choices as to which theories to believe. In this way, responsible theory choice and responsible science are not unlike responsible shopping. The big difference is that you get to go home and eat the apple and find out if you were right and it really does taste good. With theories, however, there is no feature that is the taste of truth, at least not that we get to sample. There are only features that indicate truth. You never get to go home and see unambiguously that atomic theory, for example, is true. But you do the best you can.

Theories have lots of different properties and can differ one from another in a variety of different ways. A few have been mentioned already, namely, the degree of generality and hypothetical status of a theoretical claim.