Critical rationalists hold that scientific theories and any other claims to knowledge can and should be rationally criticized, and (if they have empirical content) can and should be subjected to tests which may falsify them. Thus claims to knowledge may be contrastingly and normatively evaluated. They are either falsifiable and thus empirical (in a very broad sense), or not falsifiable and thus non-empirical. Those claims to knowledge that are potentially falsifiable can then be admitted to the body of empirical science, and then further differentiated according to whether they are retained or are later actually falsified. If retained, further differentiation may be made on the basis of how much subjection to criticism they have received, how severe such criticism has been, and how probable the theory is, with the "least" probable theory that still withstands attempts to falsify it being the one to be preferred. That it is the "least" probable theory that is to be preferred is one of the contrasting differences between critical rationalism and classical views on science, such as positivism, which holds that one should instead accept the "most" probable theory. The least probable theory is preferred because it is the one with the highest information content and most open to future falsification.