Dvojba kojom Descartes u Razmišljanjima o prvoj filozofiji želi raskrčiti put prema istini širi se od djelomične dvojbe kojom je zahvaćena vjerodostojnost pojedinih spoznaja, preko pitanja o (ne)moći spoznavanja kao takvog, do krajnjih granica osjetivih, iskusivih, zamislivih, mnijetljivih i naposljetku do subjekta samog. Pokušaj izvođenja takve posvemašnje dvojbe uključuje i zlog genija, obmanjivača, koji se kao mogućnost pojavljuje takvim pretpostavljanjem koje je obrtanje u protivno mnijenja o najboljem Bogu, tvorcu koji može sve i izvoru istine i koje (obrtanje) spada u moć slobodne volje. Descartesovu sveopću dvojbu moguće je promatrati kao svođenje na ništa (reductio ad nihilum). Takva pak dvojba pretpostavlja neka postojeća, a time i misao o tvorbi, koja je sadržana u mnijenju o Bogu i u mnijenju o zlom geniju, a potom se otkriva i u ideji o Bogu. Time je naznačena opsegovna (objektna) strana dvojbe koja se, subjektno i pri svome kraju, kroz »ja« odlučuje suočiti s veoma moćnim i veoma lukavim obmanjivačem. Pritom se »ja« u svojem postojanju, pogledom unatrag, osvješćuje i o istini svojeg porijekla. U tekstu koji slijedi, polazeći od samih Descartesovih spisa, potražit ćemo odgovore na pitanja ima li pretpostavka obmanjivača i sama pretpostavke i koje, što Descartes podrazumijeva pod zlim genijem i zašto ga treba, odnosno zašto ga uvodi u razmatranja te kako se sanjanje i ludilo odnose prema dvojbi. In his treatise Meditations on the First Philosophy, Descartes seeks to clear the path to truth by means of doubt which extends from partial doubt, affecting the credibility of certain insights, to queries about man’s (in)capacity for reasoning as such. It pursues the ultimate limits of that which can be sensed, experienced, thought or imagined (res) and ends with the subject itself. The endeavor to pursue doubt to its limits involves the evil genius, the deceiver who, as a possibility, emerges with such inverted reasoning as to oppose notions about the goodness of God, of an omnipotent Creator who is the wellspring of truth, and which (inverted reasoning) is representative of the power of free will. It is possible to regard Descartes’ universal doubt as a reduction to nothing, »reductio ad nihilum«. Still, doubt of this kind presupposes actual existence (res existens); similarly, thoughts on creation are contained in thoughts on God and the evil genius and later are revealed in the idea of God. The objects of doubt are thus outlined, and finally doubt resolves subjectively, through »I«, to confront the very powerful and very cunning deceiver. In doing so and by looking back, »I« in its existence becomes aware of the truth of its origin. In this article we will attempt, by analyzing Descartes’ texts, to resolve whether the assumption of the deceiver contains within it assumptions as well and the nature thereof; also, who is the evil genius for Descartes, why does he need him, what is the reason for including him in his reflections, and finally, in what way do dreaming and madness relate to doubt?