Senses

I don't understand how Descartes was bale to go over his arguments without senses and by simply just thinking. I however do understand how the senses can deceive you. Sometimes dreams seem so real that when I wake up, I'm confused as to whether or not I'm dreaming now or whether I was dreaming before. But that requires thought process too. To determine whether life is the dream, or whether the actual dream was a dream. So if thinking just as deceiving as the senses? I guess in my mind there is no way that it is possible for there not to be any doubt, depending on the situation. But then again, since I said depending on the situation, I'm also saying I'm wrong. I'm not sure if there is a way to not include the senses or thinking without someone having a doubt as to whether or not your claims are reliable.