I think; therefore I am. And not even a demon, a great imagined threat could disturb this axiom. I am aware of my thoughts, the consciousness of the self thinking something, and even if such thoughts are unclear, delusional and false they are my thoughts none the less; I must be real if I am creating such thoughts regardless of their status.
But a demon is not real as it is a hypothetical creature borrowed loosely from various myths and religious cosmologies. The imagined threat is never as visceral, real and brutal as what comes to be in the actual world.
Descartes was mentally incapable to imagine a real world where all truths and reality itself could be stripped to the bone. Such is the great failure of any single person's imagination in the formation of grand philosophical systems or any cartography of the world. Reality, the potential for the new, the building of the future, the ceaseless manipulation of the past that contracts and expands falling apart in our hands is the work of something far beyond the insignificance of the individual's imagination. Imagination releases thoughts like semen into the world, absolutely worthless, to be discarded like waste and filth; it takes the seminal energies of humanity, in a dynamic complex, and the fertile womb of the physical world that, itself too, interacts with such a complex of thoughts aborting and nurturing at random.
The simple imagination of one man was far from the real threat that could actually manifest itself far beyond the potential of any one individual imagination. "Gott ist tot," Nietzsche declared through the character of the madman in the market place; he wanted us all to realize the death of God in the Western world, and foresaw, in his limited ability to conceive, a world of nihilistic brutality. His individual imagination, though capturing the horror of a Godless world, was incapable of imagining the level at which such a dynamic of human energy and a fertile womb could create real history. Endless blood and semen of hundreds of millions lubricating the engines of tanks and planes in world wars, and ovens igniting flesh, boiling fat and covering everything in a burnt gristle.
Descartes, and his individual imagination, could not conceive of a world where everything could sincerely be doubted. He is almost as a jester with the threat of the demon that acts more as an annoyance to be overcome rather than a threat that leaves us overcome with anxiety -- what simplicity in retrospect!
The world we live in now is a world where the real is absolutely impossible. The demon never revealed himself to try and rob us of our reality and remains just a hypothetical for young philosophers to counter, allowing them access to the world, nominally in petty argument. Technology is the "demon" that Descartes could not conceive as an individual of limited imagination; far more pernicious and all-encompassing, a reality of something that was possible for the humbler forms of humanity living in relation to a natural world, brought to death and life, brought to reality through the breath and encompassing of the forest, dessert or ocean.
Now we are in a world that is fed to us by technological production. All senses have been analyzed, all faculties considered, and all of our products are made to plug them all up and satisfy them in deprivation chambers of satiety. Most of these things though are ideological manifestations of things rather than the real things themselves, that still remain in our collective minds from our roots of a humanity long passed. A tomato was once real, we knew all of his components, how it grew, where it grew -- we grew it ourselves, watching the slow, natural process and we consumed thankfully. Now tomatoes are engineered in a genetics lab to attempt to capture this confused, notional, nominal conception of a tomato, grown in inconceivable fields of endless, homogenized crops, covered in pesticides that kill and stop all interactions with the world, fertilized in engineered fertilizers that allow crops to grow on soil with no nutrients, ripened in chambers with ethylene gas, grouped together with tomatoes from a hundred other farms, shipped across oceans and countries, sometimes frozen, thawed and delivered to the grocery store to sit on a shelf so you can grab it thoughtlessly -- "oh, a tomato, I need that." That tomato in the consumerist capitalist world may as well be a hallucination of a schizophrenic. The process by which it has arrived to you is so abnormal, and robs it of most of the nutrients and the qualities that make a tomato real to a human body, that this tomato you have is just satiating your old notion of what a tomato. You had nothing to do with its creation, you have no idea how it was created, what is the natural process by which a tomato is created, you are blind, dumb and deaf to the world and are in a deprivation chamber of satiety. The only role you play in this "world" is to continue to believe that it is real, and that you desire to be sated by it so you participate in it, and give it your seminal energy, your blood and the only way you can show value for anything in this system is by giving your money to whomever needs it to create more contraptions to stick in your orifices.
What does it matter that you know you exist -- Cogito ergo sum? I think; therefore I am. I know I exist but I exist in this "demon's" world no longer as a hypothesis!