What does Descartes think he discovers that is beyond any possible doubt, some truths that cannot be challenged -- even if so much that people had taken for granted as true in science, religion, and the whole culture had been seriously challenged and even rejected

“I exist”

Explanation:

In his attempt to prove the existence of God, Rene Descartes a French philosopher seeks to find a permanent validity for clear and unique perceptions, he then ultimately discovers that “I exist” to be impossible to doubt and is, therefore, completely valid.

Hence, in this case, the correct answer is the philosophical phrase "I EXIST" which withstands the philosophical test of logical doubt.

answer; people have no objective standard by which to judge themselves;