For me Heidegger has always been the essential philosopher…My entire philosophical development was determined by my reading of Heidegger - Michel Foucault
In the year 1639, in his book Discourse on the Method, French philosopher Rene Descartes penned the famous line: Cogito, ergo sum or "I think, therefore I am."
It is a line which might have read I doubt, therefore I am, given that Descartes was founding his proof of existence on radical doubt. His point was that if I'm here to doubt my own existence, I at least confirm that I must really be here to do so. There must be a definite mind executing the task. So far, so good. Existence and consciousness are now proven. Ureka! What a relief.
However, if we dare take another look at the famous line, we might ponder what is really so great about it. Although it transformed philosophy, and instigated what became known as "Cartesian Dualism," what does it actually convey? Not very much. It doesn't tell us what thinking is or what it means to "be." Like so many statements, it leaves out more than it affirms.

In fact, it's no easy matter finding answers to what it means to "be." It's far easier to tackle the act of thinking. Martin Heidegger agreed, saying that the whole question of Being fell out of concern thousands of years ago, when thinkers such as Plato and Aristotle decided that Being was clearer, as a concept, when it is re-defined and equated with "beings" in the ordinary sense.
This conflation allowed them to focus not on Being itself, but on the phenomena available to our senses, namely the objects and entities all around us. This includes human beings. Note the last word - beings. Here we are again.
The ancient Greek word for being is onto, which turns up in the term ontology, denoting the branch of philosophy dedicated to the study of Being.
In English, the problem can be summarized by thinking about the verb "is." When one says that the tree is in the garden, that it is raining, that a friend is coming over, or that this is the truth, what exactly is meant by "is?" What does it allude to? After a little thought, we realize it alludes to existence.
A helpful analogy, when puzzling over Being, concerns the nature of light. Although it illuminates everything we see, it remains unseen and outside normal mental attention. It is very definitely invisible by proximity. This oddity is an essential consideration for anyone investigating the elusiveness of Being.
An easier way of grasping the quarry is to think of any background. Take, for example, the background behind anything seen, the background of everything. All objects and entities sit on something in order to exist and be in relationship with other things. A vase sits on a coaster, which sits on a table standing in the room. Where does the room sit? It's part of a residence standing on the ground, which stands upon the land in a country which is part of a continent standing upon the earth. The earth, in turn, is part of a solar system and galaxy, which are part of the universe.
But on what or in what does the universe, with everything in it sit? Answers on a postcard please?
The key point is that the background - known to Platonists as the ''Khora'' - is rarely if ever seen or considered. It's not included in everyday sensual apprehension, even though it is most certainly there. Some kind of nonconscious editing is at work it seems.