Since we began life as infants, and made various judgements concerning the things that can be perceived by the senses before we had the full use of our reason, there are many preconceived opinions that keep us from knowledge of the truth.
This statement, which is the first sentence in the entire text, can be read as Descartes' call to battle. He was about to present a physics that stands in stark contradiction to everything our senses tell us, and so it was crucial for him to undermine our faith in all the sensory-influenced beliefs that we have acquired up until now as well as in the power of our senses to ever tell us anything certain. To this end, he developed an ingenious opening gambit. He asks us to throw out all of our old beliefs, to doubt everything we have ever thought we knew, and to start from scratch, building up a completely new set of beliefs based only on what is completely certain. By doing this, he is able to clear away our prejudices and open us up to his surprising and counter-sensory theory of the world.