Why being? The term 'being' enters into a variety of contrasts. It contrasts, in the first place, with 'knowledge' and with 'science'. Many philosophers in Heidegger's day and earlier, especially those who claimed to follow Kant, were concerned mainly with epistemology or the theory of knowledge, asking such questions as 'What can we know?' and 'What are the foundations of the sciences?' Heidegger was averse to epistemology: it 'continually sharpens the knife but never gets round to cutting' ( lviii. 4). Knowledge, especially the systematic knowledge of science, involves a relation, knowing, between a knower, on the one hand, and an object, or range of objects, known about, on the other. Heidegger's doubts about epistemology concern each of the three elements.
Take first the knower. What is it? Is it a pure subject wholly absorbed in the disinterested, theoretical knowledge of its subject-matter, or is it an interested human being, situated in a particular place and a particular time, with many other relations and attitudes to many other things than the objects of its science? Take secondly the relation of knowing. Why knowing? Knowing is only one relation among many that we may take up to the things of the world; it is not the first relation we adopt towards them, it is taken up fairly late in one's career, and then only sporadically; nor is it the most obvious attitude to take towards, say, one's spouse or the key to one's own front door.