Admittedly, I haven't read the whole of this book. However, I have read large pieces of it.

This is Heidegger's attempt to define being, which is a topic left undiscussed since Plato and Aristotle back in the opening moments of western philosophy. Interestingly enough, he never comes to a conclusion on this. However, he discusses humanity a lot and what being is to humans because, afterall, we use 'being' everyday and must have some concept of what it is to be. His conclusion is one that the Existentialists loved and utilized extensively. He concludes (having digressed from his original topic) that humans have no inherent nature like other animals. Instead, we create our own nature through the decisions that we make. We are the only beings that ask what and how we should be.

This read can be extremely dense and difficult. A knowledge of phenomenology and cartesian philosophy (and, maybe, some history of philosophy) is necessary. However, its knowledge, its response to Cartesian thought, and its inspiration to Existentialism makes is a very worthwhile read.