This thesis is an attempt to determine the conceptual development of the three-nature-theory of the Yogacara school of Mahayana Buddhism during the 2:nd to the 5:th centuries C.E. Seven texts have been used as sources, namely, in an assumed chronological order: Samdhinirmocanasutra, La nkavatarasutra, Madhyantavibhaga, Mahayanasutralamkara, Mahayanasamgraha, Trisvabhavanirdesa and Trimsika. The relative chronological position of each text has been established by reference to tradition, style, quotations and consensus. As far as possible, the distribution of textual passages referring to the three-nature-theory in each text has been schematically displayed. These passages have been arranged thematically so as to make possible an analysis of each nature separately. An assessment is made of the measure of conceptual development of the three-nature-theory as expressed by each text by the use of four criteria. These are, briefly: (a) The measure of elaboration of each nature; (b) The measure of integration with pan-Buddhist dogma in general and Mahayanistic dogma in particular; (c) The measure of integration with other Yogacara tenets and (d) The measure of logical coherence and elaboration of the internal formal structure of the three-nature-theory. Finally, the development of the three-nature-theory is determined by comparing its expressions in all seven texts. It may be said that the elaboration of each nature is steadily rising as time goes with the expected exception of the consummated nature the elaboration of which is constantly quite high. As for the measure of integration of the three-nature-theory with Buddhist dogma a general degree of development is certain with peaks noted for the Mahayanasamgraha and the Trimsika while the Trisvabhavanirdesa displays a lower degree of integration. The same general measure of development is noted for the integration of three-nature-theory with the Yogacara system though the picture is a bit different. The Samdhinirmocanasutra shows none and the Madhyantavibhaga and the Mahayanasutralamkara to a lesser degree than the Lankavatarasutra. The Mahayanasamgraha and the two latter texts all exhibit a high level of integration. As for the internal structure of the three-nature-theory, again it becomes more coherent and elaborated. This is notable from the Samdhinirmocanasutra and onwards until the Mahayanasutralamkara which is markedly more frugal in its exhibition of the internal relations of the three natures. The Mahayanasamgraha and the Trisvabhavanirdesa show a high degree of elaboration while this is lessened in the Trimsika.
