Descartes presented to the world the possibility of a malicious, deceiving demon as the ultimate argument for skepticism. Who is this demon?
I shall countenance two readings. On the first, the demon is the vehicle for Descartes’ skeptical point. It functions as a spring that lifts Descartes from a merely partial to a total skepticism. Call this the bloodless reading. As Aristotle notes about certain of the bloodless animals, they may be chopped into pieces and yet the parts may lead an independent existence. “But some bloodless animals and polypods can live a long time, if divided, in each of the severed parts, and can move in the same way as before they were dismembered. Examples are what is termed the centipede and other insects that are long in shape; for even the hinder portion of all these goes on progressing in the same direction as the fore-part.”  Descartes’ demon argument is a centipede, it may be chopped into pieces, analyzed, dissected, sewn back together, and still each part moves forward as before, until there is an endless literature of such centipedes. At some point it becomes unrecognizable that a demon was ever involved; perhaps now an envatted brain takes its place. It makes no difference, the centipede-parts crawl ever forward. So there is the bloodless reading. I do not wish to countenance it any longer.
The blooded reading is the more interesting of the two; let me revert to the great chain of being and call it the higher form. Here we take very seriously that it is a demon that disturbs Descartes in his meditations; we do not try to chop it out for fear that all life shall expire from the argument. For it is an emotional argument, motivated by fear—let us not make it pure and disinterested. Who is this demon, who gives Descartes such a fright?
We shall have to know a bit about demons. One sort of demon is the possessing sort. Demon possession is an involuntary interaction between a possessed human and a possessing demon; it ends with an exorcism which casts out the demon.  Claims of such possession were often fraudulent, so criteria for recognizing legitimate possession were required: (1) an ability to understand foreign languages, (2) feats of unnatural strength, (3) acting horrified in the presence of sacred objects, (4) seizures and convulsions, (5) clairvoyance. [2,3]
Descartes’ demon is not this sort of demon. His demon is malignant and deceitful, to be sure, but those demons, while they might deceive a poor girl into taking a demon for her deceased grandfather, would not deceive her about everything.  Demon possession was, by contrast, associated with the possession of impossible knowledge, not skepticism.
Let us look elsewhere for a model, then. Giordano Bruno offers a useful threefold taxonomy of harmful demons.  First are the deaf and dumb demons, which lack reason. They cannot hear or otherwise perceive threats or prayers; instead they are like brute animals. They cause injury without reason and without malice. Because they lack language, they cannot be banished, but they may be controlled by the proper ascetic techniques. Second are fearful, suspicious demons, which have language, but are unable to distinguish the possible from the impossible. They resemble the human disturbed by dreams—recall the steps of Descartes’ ladder to skepticism!—and may be banished with threats of death and fire. Lastly, there are the hateful demons. These demons are wise and reside in pure air, and they “freely distort all these things and play with humans by counterfeiting illusions of fear, anger, religion and such things. They understand languages and the sciences, but never make any firm assertions.” The result is no surprise: “And so these hateful demons introduce confusion and doubt into the human mind and senses.”
Now we have the tools to classify Descartes’ demon: it is the third type, a hateful demon, one that delights in deception in all matters. The possibility of possession by such a demon is sufficient to drive Descartes to skepticism, if only for a time—only until he banishes the demon. And by ‘possibility’ I do mean possibility, and not mere conceivability: Descartes is not merely countenancing a logical possibility, but a very real, very frightful possibility. Demons, in those days, were quite real. Incidentally, Bruno says nothing about banishing such demons. We seem to be left up to their whims. Descartes succeeds in banishing his demon only with the help of God—though we might wonder whether he succeeds, for, if we trust Bruno, hateful demons are adept at distortion even in matters of religion.
We could stop there; perhaps it would be wise to do so. But, at the risk of myself falling prey to a hateful demon who would deceive me in matters of religion, I want to raise a doubt. This doubt arises when we consider how Descartes introduces his demon. Descartes first considers that God might provide for him always to be in error. But surely, Descartes says, God would not do so, and so Descartes instead imagines an exceedingly potent demon. Very well. But Descartes says more: he says of this malicious deceiver that it gives him his being. But this is just the role of God: to create and sustain being. Descartes’ demon seems to be doing more than mere merciless toying with God’s creation. The demon itself appears as the creator. And if this is so, then Descartes’ worry runs deeper than the worry that he is possessed by a hateful demon. His worry is: What if God himself is demonic?
Seen as a hateful demon, Descartes’ deceiver gives us skepticism, but a skepticism that may be banished by God—putting aside for now our worries. I do not oppose this reading. But lurking beneath it, prowling the shadows, is a more sinister threat, that the very God who is supposed to save us from skepticism might be a demon—and now what hope is left? Who can banish God? This, then, is a skepticism that haunts the margins of skepticism itself. Descartes quickly stifles it in his text, but what does this God-demon care for his words? What recourse is there when it possesses us?
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 Aristotle. 1984. On the Progression of Animals. Trans. A. S. L. Farquharson. In The Complete Works of Aristotle: the revised Oxford translation. Ed. Jonathon Barnes. Princeton: Princeton University Press.
 Sluhovsky, Moshe. 1996. A Divine Apparition or Demonic Possession? Female Agency and Church Authority in Demonic Possession in Sixteenth-Century France. The Sixteenth Century Journal 27:4, 1039-1055.
 Walker, Anita and Dickerman, Edmund. 1991. “A Woman under the Influence”: A Case of Alleged Possession in Sixteenth-Century France. The Sixteenth Century Journal 22:3 535-554.
 Bruno, Giordano. 1998. On Magic. In Cause, Principle, and Unity: and essays on magic. Ed. Richard J. Blackwell and Robert de Lucca. Cambridge: Cambridge University Press.