As Heidegger makes clear from the untitled, opening page with which Being and Time begins, what is at stake in the book is the question of being. This is the question that Aristotle raised in an untitled manuscript written 2500 years ago, but which became known at a later date as the Metaphysics. For Aristotle, there is a science that investigates what he calls "being as such", without regard to any specific realms of being, eg the being of living things (biology) or the being of the natural world (physics).

Metaphysics is the area of inquiry that Aristotle himself calls "first philosophy" and which comes before anything else. It is the most abstract, universal and indefinable area of philosophy. But it is also the most fundamental.

With admirable arrogance, it is the question of being that Heidegger sets himself the task of inquiring into in Being and Time. He begins with a series of rhetorical questions: Do we have an answer to the question of the meaning of being? Not at all, he answers. But do we even experience any perplexity about this question? Not at all, Heidegger repeats. Therefore, the first and most important task of Heidegger's book is to recover our perplexity for this question of questions: Hamlet's "To be or not to be?"

For Heidegger, what defines the human being is this capacity to be perplexed by the deepest and most enigmatic of questions: Why is there something rather than nothing? So, the task of Being and Time is reawakening in us a taste for perplexity, a taste for questioning. Questioning – Heidegger will opine much later in his career – is the piety of thinking.

The first line of the text proper of Being and Time is, "We are ourselves the entities to be analysed". This is the key to the crucial concept of mineness (Jemeinigkeit), with which the book begins: if I am the being for whom being is a question – "to be or not to be" – then the question of being is mine to be, one way or another.

In what, then, does the being of being human consist? Heidegger's answer is existence (Existenz). Therefore, the question of being is to be accessed by way of what Heidegger calls "an existential analytic". But what sort of thing is human existence? It is obviously defined by time: we are creatures with a past, who move through a present and who have available to them a series of possibilities, what Heidegger calls "ways to be". Heidegger's point here is wonderfully simple: the human being is not definable by a "what", like a table or a chair, but by a "who" that is shaped by existence in time. What it means to be human is to exists with a certain past, a personal and cultural history, and by an open series of possibilities that I can seize hold of or not.

This brings us to a very important point: if the being of being human is defined by mineness, then my being is not a matter of indifference to me. A table or chair cannot recite Hamlet's soliloquy or undergo the experience of self-questioning and self-doubt that such words express. But we can.

This is the kernel of Heidegger's idea of authenticity (Eigentlichkeit), which more accurately expresses what is proper to the human being, what is its own. For Heidegger, there are two dominant modes of being human: authenticity and inauthenticity. Furthermore, we have a choice to make between these two modes: the choice is whether to be oneself or not to be oneself, to be author of oneself and self-authorising or not. Heidegger insists, as he will do throughout Being and Time, that inauthenticity does not signify a lower or lesser being, but many readers have had reason to doubt such assurances. Theodor Adorno, famously critical of Heidegger, asks: doesn't authenticity end up being a jargon that we are better off without? Let's just say that the point is moot.

Regardless of the twin modes of authenticity and inauthenticity, Heidegger insists early in Being and Time that the human being must first be presented in its indifferent character, prior to any choice to be authentic or not. In words that soon become a mantra in the book, Heidegger seeks to describe the human being as it presented "most closely and mostly" (Zunächst und Zumeist).

Note the radical nature of this initial move: philosophy is not some otherworldly speculation as to whether the external world exists or whether the other human-looking creatures around me are really human and not robots or some such. Rather, philosophy begins with the description – what Heidegger calls "phenomenology" – of human beings in their average everyday existence. It seeks to derive certain common structures from that everydayness.

But we should note the difficult of the task that Heidegger has set himself. That which is closest and most obvious to us is fiendishly difficult to describe. Nothing is closer to me than myself in my average, indifferent everyday existence, but how to describe this? Heidegger was fond of quoting St Augustine's Confessions, when the latter writes, "Assuredly I labour here and I labour within myself; I have become to myself a land of trouble and inordinate sweat." Heidegger indeed means trouble and one often sweats through these pages. But the moments of revelation are breathtaking in their obviousness.