In the 1920s, Heidegger's philosophical work centered on the question concerning the meaning of being. What does it mean (for anything at all) to be? By about 1936, his thoughts had moved to a different question concerning what he now called the truth of being, by which he meant the manifestation – or event (Ereignis) – of being as such, with whatever meaning it might have in different epochs of Western history. The “history of being” is the history of the human understandings of being that have grounded Western thought and culture from Greek antiquity, exemplified especially in ancient philosophy and literature, to the modern age, which is defined by the rise of experimental science and machine technology. It has always been the hidden task of great thinkers, Heidegger argues, to articulate those understandings of being and to bear witness to the unspoken – indeed, unthought – happening of the truth of being.