Once again, I am faced with the humbling task of reviewing one of the most significant works in the history of Western philosophy. Any reader would do well to search the literature for commentary by philosophers with far greater expertise than my meager credentials, particularly regarding Heidegger's Being and Time. I will content myself, however, with recording my reactions to and impressions of this amazing work. I'm astonished that it has taken me nearly four decades of studying philosophy to read it.
Heidegger claimed to be making a radical break from everything that had preceded him in the history of philosophy, and while his work is certainly novel and quite original, one can usefully connect him to previous philosophical currents. Roughly speaking, Heidegger seems to stand to Husserl as Kant stands to Descartes. Descartes project was to establish how and what we can know about ourselves, God, and especially the external world. Through a process of introspection and rational examination of our ideas, one can be sure both that God exists and that we can have knowledge of an external world if we simply employ a careful epistemological method. Kant was dissatisfied with these conclusions and with all of the conclusions of the speculative philosophy of his time and inaugurated what he called a "Copernican revolution" in philosophy. This involved recognizing that our understanding of the world is constrained or structured by our own cognitive nature. The world as we experience it is as we must experience it, because we intuit it in the forms space and time. We furthermore, understand it according to specific "categories of understanding" which establish the form of our experience.
Fast forwarding to the twentieth century, Husserl picked up the Cartesian project. Husserl sought to establish a foundation for all human knowledge. His approach to this changed over time, but an important period of his work involved advocating for the method of the "transcendental-phenomenological reduction" by which one reflects on the world in a manner that goes beyond our ordinary apprehension of it. Through a processes or faculty he calls "eidetic intuition," one comes to understand the world as it truly is.
Heidegger dedicated Being and Time "in friendship and admiration" to Husserl, so we should think seriously about how this work compares with Husserl's philosophy. This is where the analogy between Descartes and Kant on the one hand and Husserl and Heidegger on the other is illuminating. As Kant recognizes that philosophical speculation about a transcendent reality must be limited by our capacities for experience, Heidegger recognizes that penetrating to the essences of things in the world steps beyond our experience. The central idea in Heidegger's thought is "Dasein" or human existence. It is the object and constraint of Heidegger's phenomenological investigations in Being and Time. Directly translated into English, "Dasein" might be understood as "being-there." Dasein involves being thrown into a world of experience which is in large part constituted by our capacity to care. Our world is one of our particular projects and goals. The things in this world are "ready-to-hand" as tools are to workers. In this respect, Heidegger is truly a philosopher for the industrial age. Human experience largely is fashioned out of our interaction with our technology. At the same time, Heidegger explores "being-with-others," "being-towards-the-end," and "being-towards-death."
Along the way, Heidegger discusses a variety of psychological states as modes of experience, including fear, anxiety, and curiosity. These reveal the nature of our being in the world. He is most interested in discovering the fundamental nature of being. This is not revealed in language nor in everyday being, though they are not irrelevant to a full understanding of being, but through attention to the ontic structure being. The ontic structure of being must be distinguished from the ontological structure of being in that the latter pertains to entities, while the former pertains to being itself. To plumb these depths, Heidegger emphasizes the role of care in our being and its foundation in temporality. Ultimately, understanding temporality plays the most significant role in understanding the nature of Dasein.
A humble warning: I doubt my own understanding of Heidegger. In any case, Heidegger is a notoriously difficult philosopher to understand; largely, because of how ambitious his project is, but also because of his desire to explain what he believes cannot be explained by the language as it had thus far been developed. Consequently, he employs a unique jargon which is an obstacle to understanding. Many philosophers have suggested that he is intentionally obscure to hide a paucity of arguments. I suspect this is too cynical. Instead, Heidegger seems to me to be making a conscientious attempt to describe a vision of being that goes well beyond the analytical approach that Kant employed in his metaphysics of human experience. Heidegger not only describes the basic structure of appearances, but he attempts to set human experience in motion and describe it in much richer detail than what we see in Kant's critical philosophy. Whether or not Heidegger is successful does not detract from his success in prompting many philosophers, myself included, to think about human existence in a deeper way -- in a way that connects psychology and ontology. His reputation as possibly the greatest twentieth century philosophy is not undeserved.