This study deals principally with the so-called doctrine of illumination of St. Bonaventure. The emphasis is laid on the affinity of Bonaventure's doctrine with Heidegger's ontological difference. According to Bonaventure, human knowledge has a twofold source. The distinct and clear content of all knowledge stems from experience. The absolute character of knowledge however is due to the always present background of the absolute itself which surpasses all finiteness. In contrast to a modern tendency to stress the historicity of the ultimate horizon of the spirit, Bonaventure emphasises the surpassing of finiteness and temporality. The question which arises as a result of this contrast is whether the ontological difference remains when the radical otherness of each finiteness is denied.