Vollmer thinks of the hypothetical-deductive model as involving a two-level structure. On the lower level is a set of empirical laws connecting observable features of phenomena. These laws provide the backing for causal explanations of particular events. They in turn are explained by theoretical hypotheses couched in terms of nonobservable events, characteristics, or whatever. Apart from a few errors, most notably the suggestion that empirical laws can be deduced from theoretical principles alone (without connecting bridge hypotheses), this does represent a pattern that is found in a few highly developed sciences and that has undoubtedly exercised a powerful fascination for less developed sciences, not excluding psychology.