The First Meditation, subtitled "What can be called into doubt," opens with the Meditator reflecting on the number of falsehoods he has believed during his life and on the subsequent faultiness of the body of knowledge he has built up from these falsehoods. He has resolved to sweep away all he thinks he knows and to start again from the foundations, building up his knowledge once more on more certain grounds. He has seated himself alone, by the fire, free of all worries so that he can demolish his former opinions with care.
The Meditator reasons that he need only find some reason to doubt his present opinions in order to prompt him to seek sturdier foundations for his knowledge. Rather than doubt every one of his opinions individually, he reasons that he might cast them all into doubt if he can doubt the foundations and basic principles upon which his opinions are founded.
Everything that the Meditator has accepted as most true he has come to learn from or through his senses. He acknowledges that sometimes the senses can deceive, but only with respect to objects that are very small or far away, and that our sensory knowledge on the whole is quite sturdy. The Meditator acknowledges that insane people might be more deceived, but that he is clearly not one of them and needn't worry himself about that.
However, the Meditator realizes that he is often convinced when he is dreaming that he is sensing real objects. He feels certain that he is awake and sitting by the fire, but reflects that often he has dreamed this very sort of thing and been wholly convinced by it. Though his present sensations may be dream images, he suggests that even dream images are drawn from waking experience, much like paintings in that respect. Even when a painter creates an imaginary creature, like a mermaid, the composite parts are drawn from real things--women and fish, in the case of a mermaid. And even when a painter creates something entirely new, at least the colors in the painting are drawn from real experience. Thus, the Meditator concludes, though he can doubt composite things, he cannot doubt the simple and universal parts from which they are constructed like shape, quantity, size, time, etc. While we can doubt studies based on composite things, like medicine, astronomy, or physics, he concludes that we cannot doubt studies based on simple things, like arithmetic and geometry.
On further reflection, the Meditator realizes that even simple things can be doubted. Omnipotent God could make even our conception of mathematics false. One might argue that God is supremely good and would not lead him to believe falsely all these things. But by this reasoning we should think that God would not deceive him with regard to anything, and yet this is clearly not true. If we suppose there is no God, then there is even greater likelihood of being deceived, since our imperfect senses would not have been created by a perfect being.
The Meditator finds it almost impossible to keep his habitual opinions and assumptions out of his head, try as he might. He resolves to pretend that these opinions are totally false and imaginary in order to counter-balance his habitual way of thinking. He supposes that not God, but some evil demon has committed itself to deceiving him so that everything he thinks he knows is false. By doubting everything, he can at least be sure not to be misled into falsehood by this demon.