However...

However, skepticism could not prevail in philosophy, and reason mounted the throne offered to it by Aristotle, until skepticism emerged again in the 16th century in an atmosphere of doubt and rebellion against the authority of reason. Descartes emerged in this atmosphere and he tried to bring back certitude to philosophy. Descartes: Descartes (1596-1650) began his philosophy with sweeping doubt. Ideas, he reasoned are susceptible to error and sense perception is often deceptive.

The point of departure for philosophical certitude was the existence of his thoughts, which leads him to infer his own existence: 'I think, therefore, I am'. This statement is true because it is clear and distinct. He therefore adopts as a general rule the principle that all things that we conceive very clearly and distinctly are true. Ideas seem to be of three sorts: (1) those that are innate, (2) those that are foreign and come from without, (3) those that are the mind's constructs.

Descartes disposes of skepticism by first proving the existence of God, whose idea belongs to the first class. Since we as imperfect beings are not sufficient reason for the idea of perfection we entertain the idea of God being the idea of an absolutely perfect being the idea of God must have been caused by Him. God is thus the first objective reality posited by Descartes.

Now since God is good, the innate ideas (which include the ideas of external bodies) which we have such strong inclination to believe must be true. This is how Descartes posits external reality and the possibility of science. Al-Sadr points out that 'I think, therefore, I am', contains a concealed syllogism: 'I think, every thinker exists, therefore I exist'.

Moreover as pointed out by Ibn Sina, this argument from thought to existence is invalid; for the thinking subject admits his existence in the first phrase 'I think'. Secondly, Descartes confuses between the idea of a perfect being and the objective reality it represents. It is God, not the idea of God, which is more sublime than human beings. Descartes bases the whole edifice of existence on the proposition: "It is impossible for God to deceive".

He confuses between 'deception is impossible', and 'deception is abominable', which is not a metaphysical (judgement of fact) but an ethical (judgement of value) proposition. In any case, the author's purpose is not an elaborate criticism of Descartes' philosophy but to present his view regarding the possibility of knowledge.