The evil demon, also known as evil genius, and occasionally as malicious demon or genius malignus, is a concept in Cartesian philosophy. In his 1641 Meditations on First Philosophy, René Descartes hypothesized the existence of an evil demon, a personification who is "as clever and deceitful as he is powerful, who has directed his entire effort to misleading me." The evil demon presents a complete illusion of an external world, including other minds, to Descartes' senses, where there is no such external world in existence. The evil demon also presents to Descartes' senses a complete illusion of his own body, including all bodily sensations. Some Cartesian scholars opine that the demon is also omnipotent, and thus capable of altering mathematics and the fundamentals of logic, though omnipotence of the evil demon would be contrary to Descartes' hypothesis, as he rebuked accusations of the evil demon having omnipotence.
Another such method of systematic doubt is the deus deceptor (French dieu trompeur), the "deceptive god". Cartesian scholars differ in their opinions as to whether the deus deceptor and the evil demon are one and the same. Among the accusations of blasphemy made against Descartes by Protestants was that he was positing an omnipotent malevolent God.
Kennington states that the evil demon is never declared by Descartes to be omnipotent, merely to be not less powerful than he is necessarily deceitful, and thus not explicitly an equivalent to an omnipotent God. The evil demon is capable of simulating an external world and bodily sensations, but incapable of rendering dubious things that are independent of trust in the senses, such as pure mathematics, eternal truths, and the principle of contradiction.
However, this was not the view of Descartes' contemporaries. Voetius accused Descartes of blasphemy in 1643. Jacques Triglandius and Jacobus Revius, theologians at Leiden University, made similar accusations in 1647, accusing Descartes of "hold[ing] God to be a deceiver", a position that they stated to be "contrary to the glory of God". Descartes was threatened with having his views condemned by a synod, but this was prevented by the intercession of the Prince of Orange (at the request of the French Ambassador Servien).
The accusations referenced a passage in the First Meditation where Descartes stated that he supposed not an optimal God but rather an evil demon "summe potens & callidus" (translated as "most highly powerful and cunning"). The accusers identified Descartes' concept of a deus deceptor with his concept of an evil demon, stating that only an omnipotent God is "summe potens" and that describing the evil demon as such thus demonstrated the identity. Descartes' response to the accusations was that in that passage he had been expressly distinguishing between "the supremely good God, the source of truth, on the one hand, and the malicious demon on the other". He did not directly rebut the charge of implying that the evil demon was omnipotent, but asserted that simply describing something with "some attribute that in reality belongs only to God" does not mean that that something is being held to actually be a supreme God.
That the evil demon is omnipotent, Christian doctrine and Descartes' denial of that accusation notwithstanding, is seen as a key requirement for Descartes' argument by Cartesian scholars such as Ferdinand Alquié, Beck, Émile Bréhier, Chevalier, Frankfurt, Étienne Gilson, Anthony Kenny, Laporte, Kemp-Smith, and Wilson. The progression through the First Meditation, leading to the introduction of the concept of the evil genius at the end, is to introduce various categories into the set of dubitables, such as mathematics (i.e. Descartes' addition of 2 and 3 and counting the sides of a square). Although the hypothetical evil genius is never stated to be one and the same as the hypothetical "deus deceptor," (deceptive god) the inference by the reader that they are is a natural one, and the requirement that the deceiver is capable of introducing deception even into mathematics is seen by commentators as a necessary part of Descartes' argument. Kenney exemplifies Cartesian scholarship on this point, stating that the reason that Descartes introduces a second hypothetical, beyond the original hypothetical of the deus deceptor, is that it is simply "less offensive. The content of the two hypotheses is the same, namely that an omnipotent deceiver is trying to deceive." Scholars contend that in fact Descartes was not introducing a new hypothetical, merely couching the idea of a deceptive god in terms that would not be offensive.
Janowski points out one reason for not accepting this interpretation, the same as given by Kennington, namely that the set of things that the evil demon is stated as rendering dubious ("the heavens, the air, the earth, colours, figures, sounds, and all external things") is only a subset of the things that the deus deceptor is stated as rendering dubious (earth, heavens, extended things, figure, magnitude, place, and mathematics). The omission of mathematics implies either that the evil demon is not omnipotent or that Descartes retracted Universal Doubt. Janowski notes that in The Principles of Philosophy (I, 15) Descartes states that Universal Doubt applies even to "the demonstration of mathematics", and so concludes that either Descartes' Meditation is flawed, lacking a reason for doubting mathematics, or that the charges of blasphemy were well placed, and Descartes was supposing an omnipotent evil demon.
W. Teed Rockwell, claiming to be a Deweyan pragmatist, argues that instead of being dualists or Cartesians, "philosophers should realize that the human conscious self is not reducible to the brain, nor to the nervous system, nor even to the human body. The thinking, conscious self is a nexus--or a "behavioral field"—of the brain, the nervous system, the body, and the world." Rockwell contends that his position "can allow for solutions to certain philosophical problems such as the 'brain in a vat,' . . . a contemporary, materialist version of the problem introduced by Descartes's 'Evil Genius'". "Both thought experiments are supposed to show us that human consciousness is plausible even though there might be no world in which consciousness exists," but Rockwell argues "that even in a vat the brain would have to be stimulated by some world, if only a world of electronic gizmos, and that such a world would have to produce a continuous experience. The brain, hence, would have to be embodied in some way.
- Alan E. Musgrave (1993). Common Sense, Science and Scepticism: A Historical Introduction to the Theory of Knowledge. Cambridge University Press. p. 202. ISBN 0-521-43625-7.
- Zbigniew Janowski (2000). Cartesian Theodicy: Descartes' Quest for Certitude. Springer. pp. 62–68. ISBN 0-7923-6127-X.
- Richard Kennington (1991). "The 'Teaching of Nature' in Descartes' Soul Doctrine". In Georges Joseph Daniel Moyal. Rene Descartes: Critical Assessments. Routledge. p. 139. ISBN 0-415-02358-0.
- Richard M. Kennington (2004). "The Finitude of Descartes' Evil Genius". On Modern Origins: Essays in Early Modern Philosophy. Lexington Books. p. 146. ISBN 0-7391-0815-8.
- Baldner, Steven (2006-12-01). "Rockwell, W. Teed. Neither Brain Nor Ghost: A Nondualist Alternative to the Mind-Brain Identity Theory; Book review". The Review of Metaphysics. 60 (2): 419(3). ISSN 0034-6632.
- O. K. Bouwsma (1965). "Descartes' Evil Genius". Philosophical Essays. University of Nebraska Press. pp. 85–98. ISBN 0-8032-6225-6. — originally published as:
- Alex Gillespie (2006). Descartes’ demon: A dialogical analysis of ‘Meditations on First Philosophy.’ Theory & Psychology, 16, 761-781.
- David Frederick Haight and Marjorie A. Haight (2004). "Dialogue between Descartes and the Evil Genius". Scandal of Reason: Or Shadow of God. University Press of America. pp. 49–70. ISBN 0-7618-2725-0.
- Rockwell, W. Teed (2007). "5". Neither Brain Nor Ghost: A Nondualist Alternative to the Mind-Brain Identity Theory. MIT Press. ISBN 0-262-68167-6.