If one eliminates every personal particularity and every self- knowledge there always remains the awareness of some kind of presence. This indefinable certainty is the only certainty there is. By consequence it is the only reliable starting point for the philosophical line of thought. Nothing is certain, nothing exists on its own and nothing even can be known completely, neither by experience nor by scientific research. But everything can be understood as a result of philosophically thinking, if the thinker takes his own mysterious presence for starting certainty.
Understanding reality is quite different from knowing in a scientific way.
The first notion concerns the quality and the character of reality in general, while the second notion is a matter of quantity and composition of concrete things. So, for example, the first says something about the notion 'tree' and the second about a special tree, on a special time and place. There are many thinkers who don't agree with the idea that 'presence' is a real certainty. They argue that one has no means to verify his knowledge about himself. Without an external checkpoint such a knowledge also would be a shot in the dark.