Descartes’ Philosophical Revolution
In the history of Christian philosophy, Anselm stands as one of the key figures. His idea that one must trust God (i.e. believe) in order to understood, was a high point in the Middle Ages.
But in the 16th century, Rene Descartes turned the world upside down. In trying to establish what he thought would be an indubitable starting point for human thought, his answer was the opposite to that of St. Anselm.
Cogito ergo sum—I think, therefore I am—placed the human mind as the unquestionable starting point in all human predication. Now, God and everything else would have to be proven by the mind of man that was now unlinked from everything. The mind of man was now autonomous.
Now the problem with this statement seems hidden from Descartes. And the problem is this. His idea of cogito ergo sum is not logical. It assumes what it is supposed to prove. Consider this syllogism:
P1. I think
P2. In order to think I must exist
C. Therefore I exist.
The problem is that the conclusion ‘I exist’ is assumed in the first premise: ‘I think.’ The moment the ‘I’ word is used, it assumes existence. In order to not beg the question, therefore, the first premise ought to be: ‘There is thinking going on,’ and you cannot get from there to ‘I exist.’ So, you can’t know that ‘you think’ because you have not justified that you exist. Consider the revised syllogism: