It may remain forever a matter of debate how much truth there is in the old claim that every important thinker has essentially one fundamental idea. In the case of famous philosophers, its vindication may oblige us to summarize the “one great idea” in such broad terms as to make it almost meaningless. What can probably be claimed with more justification is that for most great minds there has been one question that guided their thinking or research. This certainly applies to Martin Heidegger, and the question that fascinated him throughout his long philosophic life can be stated simply: what is the meaning of being? Ontology, in the widest possible sense, was his main concern throughout his life. This does not mean, of course, that he was forever looking for an answer to the same old question. As his thinking evolved, the meaning of the question changed; but Heidegger to the end of his life remained convinced that the “questionability” of the Seinsfrage was the main thrust of his life's work (cf. GA i 438).