In Being and Time, Heidegger attempts to answer the question "What is being?" He tries to do this through studying the concept of dasein, a German word that roughly translates to “being there.” He uses it primarily to refer to the human being, the being who is most self consciously aware of its own existence. He then interrogates what "being there," in other words, being self-aware, means.

While Heidegger never comes to a satisfactory conclusion—he never finished the book—he makes discoveries along the way that have had a profound impact on philosophy. Therefore, going with him on his journey becomes the reader's reward.

In Being and Time, Heidegger reexamines, as mentioned above, the human being's sense of its own consciousness. Much of modern Western philosophy has been based on Descartes's idea of duality: that we as humans have a disembodied consciousness that is separate from the material world. This is called mind/body duality. Heidegger, however, rejected the Cartesian idea that selfhood (the self-aware mind) is separated from the material world. For Heidegger, the human consciousness is deeply enmeshed in and formed by the world. In other words, you really can’t think of the self-aware being outside of his or her place in time, history, and geographical location—outside of the material and embodied.

In making this move of contextualizing dasein within the material world, Heidegger famously critiques the Enlightenment idea of how beings and objects interact each other. In Enlightenment thought, Heidegger says, when we see a chair, for example, we are understood to be engaging in a complex metaphysical process to comprehend what the chair is and analyze its use value. We then use the object, which we see as utterly separate from us. Heidegger, however, thinks that that is only one mode of approaching an object, and that, in reality, we usually don’t think about things in this way. We don't constantly think at all about how we interact with the world. Famously, Heidegger notes that when a carpenter is hammering a nail, he isn’t thinking about what he is doing, for the motion is ingrained. The carpenter doesn't necessarily feel separate from the hammer and nail, and he isn't constantly contemplating them as separate objects from him.

The important point is that the only time the carpenter thinks about the hammer in a theoretical way as a separate object is when the hammer is broken and can’t do its job. We only think deeply when we have a problem with being, Heidegger argues.

Heidegger's book also explores ontotheology—arguing that the entire history of Western metaphysics is characterized by forgetting about the question of being. Each metaphysical system, he argues, has an unspoken notion of being that is accepted unquestioningly. This unquestioned, although historical and cultural, notion of being is taken for granted as transcendent (unshakable, ultimate) truth and is then used in a circular way to justify the metaphysics in question. Getting back to Descartes and his body/mind dichotomy, Heidegger in Being and Time shows that what Descartes accepts as a pure, inviolate, and transcendent truth about being is merely part of a historical Christian (time and culture bound) way of thinking about the mind and body.

Heidegger's Being and Time was greatly influenced by Nietzsche's historicizing of abstract categories. In turn, Being and Time has immensely influenced post-structuralist philosophy, making his work important not only for philosophy but for literary studies. Derrida, for example, famously said that his entire work was an interpretation of Heidegger.