Martin Heidegger begins Being & Time by saying “Our aim in the following treatise is to work out the question of the meaning of being and to do so concretely.” He means to answer the very difficult question “What is being?” As he embarks on this endeavor, he notes two reasons he thinks previous philosophers have failed in truly understanding being. “An understanding of being is already included in conceiving anything which one apprehends in entities.” Being is probably the most basic concept any human understands. And since every human understands it implicitly it is also said to be undefinable.
So much for making his task seem as hard as it can. As if a discription of being could get any harder to understand. Heidegger continues to say “being is always the being of an entity.” Being itself is not an entity, (nor is it a property of entities) but in some way every entity has being already. So, says Heidegger, the way to understand anything about being is through an examination of entities.
Among the entities which we ever encounter there is one special entity. This entity is the only entity to have any kind of problem with figuring out what being is. That entity is obviously us. He calls any instance of this kind of entity by the name “Dasein.” This special characteristic requires that we pay special attention to Dasein as we try to figure out the meaning of being. As it turns out, as we go through Being and Time we find many characteristics unique to Dasein. Dasein makes being an issue for it, Dasein has being-in-the-world as its most basic state of being, and Dasein is familiar with the world.
On page 119, Heidegger writes, “This familiarity, in turn, is constitutive for Dasein, and goes to make up Dasein’s understanding of being.” So the fact that we are familiar with the world enough to be able to get around in the world shows that we must have already some kind of tacit understanding of being. It seems Heidegger would say this, in fact, is the only way we can understand being. In this way, Heidegger’s answer to the question of the meaning of being is like Kant’s reply to Hume about how we know about causality; the fact that you are even able to formulate the question shows that you must already be capable of the answer. In fact, a necessary condition of you being able to ask your question is that you have already understood, in some way, and that you have used your understanding of the answer every day of your life.