Rene Descartes 'Meditations'

In meditations by Rene Descartes, he said he has decided to doubt everything he previously believed to be true and instead rely on only his reasoning ability starting from the scratch and building his knowledge beginning with things of which he is completely certain. He rejects the knowledge from his sense deciding that such knowledge is unreliable and open to deception so is not trustworthy. He reasons that he himself must actually exist because he is able to doubt and to think. He knows that he himself exists and that he is able to think and also trusts his ability to think. He has a clear and instinct idea about his own existence. And he is certain over this.
Also, He goes on to convince himself of God’s existence reasoning that he could
not have come up with the idea of God himself because God is not more perfect than himself. The only way he could have an idea of such a supremely perfect being is would be if God himself planted the idea in him. For this reason, God must exist. Descartes tries to defeat uncertainty by showing that if one proceeds to systematically consider the matter, there are truths which one simply cannot doubt. He does this by using a method of doubt and a method of analysis jointly. The method of doubt involves posing more and more powerful skeptical hypotheses which call into doubt classes of knowledge claims. As a seeker of the truth it’s necessary to doubt different views in other to get a clearer picture of the real fact and truth. If you wish to investigate anything logically and critically, you need a starting place. Starting by doubting all things and building from there is logical. It's like starting at zero and adding up stuff. It's easier than starting at a random number and counting from there. That's about as simply as I can put it. Descartes' first goal was to suspend judgment about any of his beliefs which were even slightly doubtful, then make a logical foray into understanding things critically and logically, without bias of emotion, societal notions of right and wrong, or truth, or even reality (a very hard thing to do).