The philosophical interest in semiotics arose out of its chief aim, the elucidation of the foundations and forms of knowledge. Since Locke and Leibniz it has been recognized that signs not only serve to present and communicate knowledge already given, but also open up certain domains of knowledge that would otherwise be inaccessible. Since the use of sign systems presupposes insight into the rule-governed construction of these systems, it is more appropriate to speak of a semiotic complementation than of a 'semiotic transformation of philosophy' (Apel). With the exception of elementary forms of knowledge, which are, however, fundamental, all knowledge rests on an interdependence of intuitive and semiotically mediated cognitions. In the contemporary philosophy of science a planifactory function joins the cognitiye function of signs. Signs serve to plan and steer actions and operations. The cybernetic sciences as a semiotic discipline have succeeded, for the first time since the breakthrough of modern science, in reversing the relation between the natural and the human sciences. A model from the human sciences has successfully been superposed upon natural sciences and technical disciplines.