The history of philosophy and science can be viewed from several perspectives. From an external point of view, concerning the so-called socio-economic conditions of theory construction, and from an internal point of view which concerns the logical structure of theory. One important internal point of view involves the effort to give a foundation to theoretical propositions which can be established by itself without recourse to any other propositions, especially not empirical ones. A proposition that possesses such a foundation, is normally called a priori. The question is whether there are a priori propositions, i.e. whether propositions in theoretical contexts can be so identified that they fulfill the cited condition of independence. In regard to the history of philosophy and science it seems as if this is still an open question; one often hears. it said that the problem is as old as philosophy itself (a formulation which usually disguises an acute theoretical weakness). Those who answer affirmatively are called rationalists; those who answer negatively empiricists.