ENThe article traces Heideggerian thought back to its pre-conceptual origin and demonstrates how thus a transformation of the notion of rationality has been achieved in his philosophy. The thinker focuses his investigation on the ontological basis of Western rationality. He also develops the structure of his own thought by reinterpreting this ontological basis as expressed in Aristotle and Plato, in particular, and the Greek thought of presence in general. Heidegger rediscovered the crucial link between thinking and finitude, hence, his early musings on pre-conceptual origins of logical, or conceptual, thought later, in his mature period, culminated in the thinking of pain as the ontological difference. Keywords: Heidegger; logos; ontology; pain; rationality. [From the publication]