Doubt is very important in Descartes' "Meditations on First Philosophy." It helps him challenge ideas he believed before and build a strong basis for real knowledge. Descartes uses a method called methodical doubt. This means he carefully questions everything he thought was true.

1. Radical Doubt: Descartes' way of thinking makes him doubt his senses. He realizes that what he sees or hears might not always be true. This makes him wonder if he can be sure of anything at all. This leads to his famous saying: “Cogito, ergo sum,” which means “I think, therefore I am.” Just by doubting, he proves that he is thinking, which shows that he exists as a thinking person.

2. Existence of God: While meditating, Descartes doesn’t just doubt the physical world; he also questions if God exists. He wonders if a powerful deceiver could be tricking him. The answer to this worry is important: if a good God exists, then He wouldn’t deceive him, and Descartes can trust that the outside world is real.

3. Knowledge Foundation: This doubt acts like a clean-up process. It allows Descartes to rebuild his knowledge on strong ground. He gets rid of what is false to find out what can really be known. By the end of his meditations, skepticism turns into certainty. This shows that doubt isn’t just harmful; it can also help build new ideas.

4. Philosophical Method: The way Descartes approaches his "Meditations" shows a method of thinking known as rationalism. Instead of relying on experiments or outside information, he focuses on inner thoughts and logic. Through doubt, Descartes creates a way to think that fits with these rationalist ideas, helping set the stage for modern philosophy.

In summary, doubt isn't just feeling unsure. It’s a useful tool that helps people search for what is certain and can lead to greater understanding.