He decided that he would assume that each of his opinions had at least "some reason for doubt". He believed that all he had acquired as perception he acquired through his senses, and he was sure through his meditations that these were unreliable.
His premise was that the senses rob people of their judgments. Descartes described the sky, the clouds, the earth itself as subject to perception of the senses. He was sure that each of us saw these objects differently so how could our senses be reliable. Descartes was skeptical of the senses as reliable, and in the Second Meditation he continued this methodology to prove that doubt could actually prove that the nature of the mind was easier to understand than the body. Common wisdom to the contrary, Descartes wanted to prove that a sense of knowledge would lead one to understand that a hand or leg is a thing but not the entire you. For instance if someone had their leg amputated and the limb lied 20 feet away on an operating table, was that limb the person or just a perception of who they were. If body parts were only a thing then who is the "I" that all of us strive to know and understand. Descartes concluded that the method of doubt employed here allowed him to understand himself. All that the imagination allowed him to perceive about the body did not help him understand himself. His best example of the senses ability to deceive is that one feels the heat in a room and the smells of the air exist, but if one is asleep does that make them all false because one is not mindful of them. He believed that whether these senses and their attending perceptions were true or false was irrelevant to understanding one's self.
His conclusion through his method of doubt is that even the body is not easily perceived by the senses of touch, smell or sight. Each of these senses is subject to an individual's perception therefore very unreliable.