Philosophy enquires into the relationship between being and thought, existence and determination. In his early works, Kant approaches this basic problem of philosophy as determined by conventional rationalist metaphysics. Starting from the realisation that existence is not the determination, is not the predicate of an object, but its absolute position, Kant intervenes in the ontological and theological discussion of his age and develops his own philosophical position from the differentiation between real and philosophical reasons. The author demonstrates Kant's struggle with this basic problem from his pre-critical writings up to his Critiques. In an excursus he deals with the posthumous works and with the return of the approach in Schelling's late philosophy and its turn to an unpredicated being.