He was searching for the ground (a thing about which we could be certain) upon which to build the structure of knowledge.
To do so he practised a method called radical doubt.
He found that everything he could sense was doubtable because sometimes he dreamt and sensed and believed what he sensed.
Also the senses sometimes play tricks; like when you put a pencil in water it appears to be bent.
Eventually he got to the point where he examined thought it-self.
Thought – or cogito – had a much broader meaning then. It meant something more akin to our word consciousness.
He thought that because I doubt I cannot doubt that I doubt therefore I must exist as a doubting thing. He then extended this to: because I experience (cogito) though I can doubt the existence and constitution of what I experience I cannot doubt that I experience.
So he conjured the cogito ergo sum. I think therefore I am.
The problem was that he didn’t subject to doubt the reasoning; the logic that enabled him to make the statement I think therefore I am.
He also didn’t examine and define the “am” which is the being, the existing.
He separated the world into 3 distinct substances (substance then meant something self-contained and not reliant upon anything other then itself in order for its existence). They were res extensa (extended substance) res cogita (thinking substance) and res infinita divine (God substance).
What he didn’t examine was the term substance. In each case when you took away from the res (substance) the extension (which is just shape), the cogito or the infinita you were left with nothing.
See substance was supposed to be the thing which had these properties. Like in language there are predicates. This apple is green; green is the predicate and the apple is that which supports the predicate. But once you took from “substance” all its predicates you were left with nothing.
This led Berkeley (one of my favourite philosophers) to make the claim “esse est percipi” (to be is to be perceived) basically that everything exists in the mind. This because all the predicates of an objects of sense (smells, tastes, touch, sounds and images) were ideas in the mind supposedly caused by the extended objects interacting with the mind. But once you took from the object all these ideas you were left with nothing.
Also there was the problem of how the different substances interacted with each other. How did a thinking thing with no shape cause an extended thing (our body) with shape to move?
The problem was none of them focused on what it meant “to be”; to exist.
Indeed accorded to Heidegger they had forgotten how to even ask the question “What is being?” which is the wrong question because it is tautological (is and being mean the same thing).
And now we have reached Heidegger whom is new to me so I’ll finish here.