The Cartesian evil demon and the impossibility of the monstrous lie

Creative Commons Atribuição 4.0 Internacional. ABSTRACT In this paper, I address the issue of whether the evil demon could have caused the idea of God. In order to determine the capabilities of the evil demon, I perform a thought experiment in which I reaffirm the conclusion that an imperfect being could have never caused an idea of perfection and infinitude, i.e., the idea of God. The article is divided into five sections and a conclusion. While the first section is introductory, the second looks at the problem of God and knowledge certainty. Elucidating how reality is gradual according to Descartes, in the third section I address the distinction between objective, formal and eminent reality. In turn, in the fourth section, I argue that if the objective reality of God exists, that is, an idea of perfection, the imperfect evil demon could have never caused it. The last section examines the reverse argument of the fourth section, viz, whether God could have caused the existence of evil and imperfection.


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18-20). In contrast, and to reach an indubitable truth, Descartes provides a standard of truth that is twofold in the Meditations. In the first Meditation, he claims that all doubtful ideas are to be considered false. The rationale for this proposal is that doubtful ideas are not reliable in so far as they may deceive us, and what has deceived us once could deceive us again. The latter, which can be named Mr. Distrust's principle, implies that it is not necessary to show that all doubtful ideas are false, since proving their falsity is technically impossible. Indeed, one would need more than a lifetime, because doubtful ideas are countless. Rather, one can show that the source of doubtful ideas is unreliable. For example, perceptual beliefs are doubtful because the senses have deceived us once. According to Mr. Distrust's principle, then, all perceptual beliefs are doubtful and, accordingly, they are considered by Descartes false, at least in the first Meditation. 1 In the third Meditation, Descartes completes his standard of truth. According to the French philosopher, In this first item of knowledge there is simply a clear and distinct perception of what I am asserting; this would not be enough to make me certain of the truth of the matter if it could ever turn out that something which perceived with such clarity and distinctness was false. So I now seem to be able to lay down as a general rule that whatever I perceive clearly and distinctly is true (Descartes, 2008, p. 24, AT VII 35, my emphasis).
Nevertheless, the main theme of the Third Meditation is not the standard of truth; rather, the main focus is upon the a posteriori proof of the existence of God. As it becomes more evident with the a priori and the a posteriori proofs of the existence of God, 2 Descartes adopts the vocabulary of scholastic philosophers such as Aquinas. Within the Summa Theologica, Aquinas distinguishes between demonstrations a priori versus those that proceed a posteriori. In particular, Aquinas asserts that, Demonstration can be made in two ways: One is through the cause, and is called "a priori," and this is to argue from what is prior absolutely. The other is through the effect, and is called a demonstration "a posteriori"; this is to argue from what is prior relatively only to us. When an effect is better known to us than its cause, from the effect we proceed to the knowledge of the cause. And from every effect the existence of its proper cause can be demonstrated, so long as its effects are better known to us; because since every effect depends upon its cause, if the effect exists, the cause must pre-exist. Hence the existence of God, in so far as it is not self-evident to us, can be demonstrated from those of His effects which are known to us (Aquinas, 1981, p. 14, I, Q. 2, Art. 3).
This passage shows that one can prove the existence of God by proceeding from the effect to the cause. But, one can prove the existence of God by proceeding from the cause to its effect too. Here I will only concentrate upon the a posteriori proof (the so-called cosmological argument), because it has key assumptions for the other sections of this paper. Even so, I only mention the core of the latter proof: the notion of a clearly perceived substance, requires existence. God's perfection can be clearly and distinctly perceived qua substance. Therefore, God must exist. In this sense, the a priori proof in the Meditations is crucial to grasp the relation between God's perfection, qua substance, and his necessary existence. In other words, the existence of God is inseparable from his being as a substance (Descartes, 2008, p. 46, AT VII 67). Or, the existence of God is included in our concept of God (Descartes, 2007, p. 197, AT VIIIa 10).
1 Surprisingly, the reader will discover in the sixth Meditation that Descartes considers that the hyperbolic doubts of the past days are laughable, especially those regarding the senses (Descartes, 2008, p. 61, AT VII 89). 2 It is worth noting that the a priori proof is a complement of the a posteriori proof, in the sense that Descartes has to rule out that the idea of God can be fictive. For this reason, in the fifth Meditation Descartes aims at proving that the idea of God necessarily implies existence. In other words, God's perfection is not separable from his existence, because the idea of a supremely perfect being is not grasped as a fiction of the intellect, but as part of his very nature (Descartes, 2008, p. 85, AT VII 120). In fact, the idea of God can be compared to mathematical truths in that such truths and the idea of God are necessarily true and eternal. I am grateful for the opportunity to clarify this point to an anonymous reviewer.
In contrast, according to the a posteriori proof, the existence of God is an effect of a cause which is more real and powerful than the very idea: God himself. The a posteriori argument, which I take from Brecher (1976, p. 418-419), can be summarized as follows: Assumptions: 3 1) Something cannot come from nothing.
2) There cannot be more reality in the effect than in the cause.
3) The objective reality of any idea will be adequate to or correspond to the formal reality of the thing of which it is the idea. Proof: 1) I have an idea of God (an infinite, eternal, unchanging, etc., substance). 2) Something must have caused that idea (ass. 1).
3) I cannot be the cause of that idea (being finite), nor any other finite thing (ass. 2). 4) So then some existent thing equally powerful to this idea must have been the cause of it, for otherwise we obtain an infinite regress (ass. 3). 5) But such a thing is God. 6) Therefore, God exists. However, some commentators on the Meditations simply assume that appealing to the existence of God is alien to the so-called order of reasons provided by the French philosopher (See for further discussion Gueroult, 1984, p. 209). As I have shown elsewhere (González, 2017), it is the very order of reasons that has a central objective, namely, the refutation of the Pyrrhonic skeptic as well as the atheist. Furthermore, it is not possible to ensure the certainty of knowledge without proving the existence of God. The Cartesian rationale is simple: if the existence of God is not proved, the evil demon could deceive us about everything, including clear and distinct ideas. That is, the evil demon could deceive us not only about ambiguous ideas (such as perceptual beliefs), but also about clear and distinct mathematical truths. For example, the evil demon could have arranged my cognitive apparatus, so I always add 2+3=6, when in fact it is 5. And, likewise, the evil demon could have arranged that I conceive triangles with more than three sides. Can the demon deceive us concerning the existence of God? (I will return to this issue in the following sections).
Even though mistakes such as 2+3=6 and the triangle with four sides are blatantly contradictory, they cannot be ruled out if the existence of God is not proved and, furthermore, if it cannot be shown that God is perfect and benevolent. Take, for example, this passage, in which Descartes suggests that, although certain truths can be clear and distinct, they can be a deceit caused by the evil demon: Yet when I turn to the things themselves which I think I perceive very clearly, I am so convinced by them that I spontaneously declare: let whoever can do so deceive me, he will never bring it about that I am nothing, so long as I continue to think I am something; or make it true at some future time that I have never existed, since it is now true that I exist; or bring it about that two and three added together are more or less than five, or anything of this kind in which I see a manifest contradiction. And since I have no cause to think that there is a God at all, any reason for doubt which depends simply on this supposition is a very slight, and so to speak, metaphysical one. But in order to remove even this slight reason for doubt, as soon as the opportunity arises, I must examine whether there is a God, and, if there is, whether he can be a deceiver. For if I do not know this, it seems that I can never be quite certain about anything else (Descartes, 2008, p. 25, AT VII 36). This explains why Descartes offers the a posteriori and the a priori proofs of the existence of God. For if knowledge is to be shown to be certain, God must exist. Moreover, if He does not exist, the evil

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demon could deceive us about everything. This has inspired Newman (2016) to put forward the socalled non-atheistic-knowledge-thesis, which holds that certainty depends upon the existence of God. In relation to the difference between the awareness of an atheist about truths versus genuine knowledge of truths, Descartes claims that: The fact that an atheist can be "clearly aware [clare cognoscere] that the three angles of a triangle are equal to two right angles" is something I do not dispute. But I maintain that this awareness [cognitionem] of his is not true knowledge [scientiam], since no act of awareness [cognitio] that can be rendered doubtful seems fit to be called knowledge [scientia]. Now since we are supposing that this individual is an atheist, he cannot be certain that he is not being deceived on matters which seem to him to be very evident (as I fully explained) (Descartes, 2008, p. 101, AT VII 141).
Thus, there is a very important difference between being clearly aware of a truth, a state in which the atheist can be, and having indefeasible knowledge about truth. The next section focuses upon the objective reality of God, and his formal reality, which contains necessary elements of his perfect existence.

Objective, formal, and eminent reality
Substances are real as independent separable things. 4 According to Descartes, reality is not binary, that is, it is not the case that reality can either be attributed or denied to something. On the contrary, there are degrees of reality. In fact, the French philosopher clarifies the way reality has various degrees as follows: VI. There are various degrees of reality or being: a substance has more reality than an accident or a mode; an infinite substance has more reality than a finite substance. Hence there is more objective reality in the idea of an infinite substance than in the idea of a finite substance (Descartes, 2008, p. 117, AT VII 165f).
He insists on the same ideas in the Third Set of Objections with Replies: […] I have also made it quite clear how reality admits of more and less. A substance is more of a thing than a mode; if there are real qualities or incomplete substances, they are things to a greater extent than modes, but to a lesser extent than complete substances; and, finally, if there is an infinite and independent substance, it is more of a thing than a finite and dependent substance. Al this is completely self-evident (Descartes, 2008, p. 130, AT VII 185).
The first type of reality of a thing is objective reality. Don Quixote has objective reality, because he exists independently, qua substance, in our intellect. Don Quixote has no existence outside our intellect, and independently from it. Thus, Don Quixote is something that is represented by an idea, i.e., a thought, or an immediate perception which makes me aware of the thought (Descartes, 2008, p. 113, AT VII 160-161).
Many things that belong to our minds are objective realities, that is, they have objective being in the intellect (See for further clarification Descartes, 2008, p. 74, AT VII 102). For example, the sun is an objective reality, even though it is a formal reality as well. When it comes to the sun's objective reality

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what counts is whether the sun exists qua substance in our minds as an idea. By contrast, when it comes to the formal reality of a thing, it needs to have existence independently from our intellect. Interestingly, the sun is an objective and a formal reality, because it exists in our intellect, but it can also exist independently from it.
On the other hand, truth lies in what can be perceived clearly and distinctly. In the case of the sun, we have two ideas. One of them comes from the senses, depicting a bright circle in the sky, which is hard to see directly with the naked eye. The other, in turn, is an idea that comes from astronomy, namely, from the reasons that make us believe that the sun is different from the bright circle in the sky. What idea of the sun is truer? Obviously, as Descartes is a rationalist, he argues for the truth of the reasons of astronomy (Descartes, 2008, p. 27, AT VII 39). That is, there is a great disparity between the idea and the object. The sun is what the astronomy determines, despite what our senses perceive. Then, there are two ideas of the sun: one that originates in our senses, which is different from the thing itself, and one that is related to the reasons of astronomy, which pins down what the thing really is. But, now that the objective reality of an entity is relatively clear, the question that arises is: what is the formal reality?
Although Descartes does not refer too many times about the term, he gives the following definition: "IV. Whatever exists in the objects of our ideas in a way which exactly corresponds to our perception of it […]" (Descartes, 2008, p. 114, AT VII 161). He also adds what follows about the eminent reality: "Something is said to exist eminently in an object when, although it does not exactly correspond to our perception of it, its greatness is such that it can fill the role of that which does so correspond" (Descartes, 2008, p. 114, AT VII 161). God, for example, is an eminent reality.
However, an example offered by Descartes clarifies the crucial difference between objective and formal reality: Descartes' image (Descartes, 2008, p. 201-202, AT VII 289). Descartes' image can either be looked in a mirror or in a painting; either way, the image is fundamentally linked to causes. More precisely, in the case of the mirror, Descartes' semblance is the cause, while in the case of the painter, the latter is the cause of the image on the canvas. Now, if Descartes' image is transmitted to other people, and their eyes and intellect, Descartes is the primary cause of the idea formed in the minds of those people, even if those minds reduce or amplify the image. What is applied to Descartes can be applied to any other external object. But, what reality can be attributed to Descartes' image?
In view of this example, it is clear that the formal reality of Descartes is a kind of substance that flows from him to other people's minds. In particular, Descartes' substance serves to fashion an idea of him in the intellect of other people. By contrast, the objective reality is "nothing but the representation or likeness of me which the idea carries, or at any rate the pattern according to which the parts of the idea are fitted together so as to represent me" (Descartes, 2008, p. 202, AT VII 290). This idea does not seem to be anything real, because it is a relation of various parts, i.e., between Descartes' parts and himself; that is, objective reality is a mode of the idea's formal reality, "in virtue of which it has taken on this particular form" (Descartes, 2008, p. 202, AT VII 290). Note that formal reality is the cause of the idea of Descartes' body (which has various parts). Since this example can be applied to any other external object, formal reality, which is a whole substance, causes the idea, viz., the objective reality of the image. Now, recall that, according to the second self-evident second assumption of the a posteriori argument, there cannot be more reality in the effect than in the cause. More precisely, as something cannot come from nothing, the effect cannot have more reality than the cause. When applied to the difference between objective and formal reality, it is clear, then, that the formal reality, Descartes himself, is the cause of the idea one only has in the intellect. But it cannot be that the objective reality causes the existence of a formal reality, since the former is a mode of the latter and, as analyzed above, modes or accidents have less reality than substances, i.e., Descartes' idea in the intellect has less reality than Descartes himself. In the case of the objective reality of God, there must be a cause of it, which has more reality than the idea in Descartes' mind. The cause is the formal reality of God, which causes the objective reality of the idea of God; thus, God exists. In this sense, the attributes of God, such as his 7/12 omniscience, omnipotence, infinitude, eternity, immutability, perfection, cannot have been caused by Descartes. Hence, the French philosopher concludes that he is not alone in this world (Descartes, 2008, p. 29, AT VII 42).
Briefly put, the idea of God is an objective reality, an idea that cannot have been caused by Descartes. Why? Because this very idea of God cannot have come from nothing, and the cause has had more reality than the effect. Thus, the very idea of God must have been caused by supreme God, who is the formal reality that contains all the elements of anything whatever. For this reason, God not only exists; He also creates and maintains everything in the world.
In the next section, I offer a thought experiment according to which the existence of God cannot be a lie of the evil demon. That is, if God did not exist, it would still have not been possible that the evil demon lied about the existence of God.
How evil the Cartesian evil demon can be?
In 1605, Miguel de Cervantes y Saavedra published The Ingenious Gentleman Don Quixote of La Mancha. Briefly, the plot revolves around a noble from La Mancha, Don Alonso Quijano, who becomes insane after reading too many chivalry romances, and not sleeping for this reason. According to the plot, Don Alonso Quijano decides to become a knight-errant to revive chivalry, as Don Quixote de la Mancha. Sancho Panza, a simple farmer who is his squire, employs an earthy wit in dealing with Don Quixote's monologues on knighthood, which were totally old-fashioned at that time. Although Don Quixote believes that he is in a knightly story, he and Sancho embark on several adventures. One of them, perhaps the most well-known, is one about the windmills who were confused with giants by Don Quixote. This is part of the dialogue between him and Sancho Panza, in the adventure of the windmills: -'Fortune disposes our affairs better than we ourselves could have desired; look yonder, friend Sancho Panza, where you may discover somewhat more than thirty monstrous giants, with whom I intend to fight, and take away all their lives: with whose spoils we will begin to enrich ourselves; for it is lawful war, and doing God good service to take away so wicked a generation from off the face of the earth.' -'What giants?' said Sancho Panza. -'Those you see yonder,' answered his master, 'with those long arms; for some of them are wont to have them almost of the length of two leagues.' -'Consider, Sir,' answered Sancho, 'that those which appear yonder, are not giants, but windmills; and what seem to be arms are the sails, which, whirled about by the wind, make the millstone go.' (Cervantes, 2008, p. 59).
The adventure, which is hilarious, depicts Don Quixote's crash with the sails of a windmill. Owing to the crash, Don Quixote rolls over the plain. After being told off by Sancho, he retorts that an evil sage named Freston metamorphosed the giants into windmills, to deprive his glory of vanquishing the giants. The sage is regarded as mighty by Don Quixote, as he can supposedly deceive him and Sancho in many different senses. Did Descartes read Don Quixote and the story of the evil sage who supposedly enchants Don Alonso Quijano occasionally? Does the sage, Freston, have similar powers than the Cartesian evil demon? Although it is likely that Descartes knew Cervantes' novel (Cf. Ihrie, 1982, Cascardi, 1984and, especially, Wagschal, 2012, it seems that Freston and the Cartesian evil demon have different powers. While the former intervenes reality mostly with regard to time and space (Allen, 1969), changing certain scenarios, the latter exercises his power in relation to Descartes' senses, memory and, especially, to beliefs (Nadler, 1997). For this reason, I regard the Cartesian evil demon as a liar: he distorts reality as though Descartes were in a dream. All beliefs are to be considered unreliable due to the evil demon's powers.
Descartes' evil demon is the main role of a thought experiment aimed to find out whether the indubitable truth does exist. According to this thought experiment, he supposes that no God exists, but an evil demon who deceives us about everything. As such, the evil demon is a liar, a deceiver of a supreme power, who deliberately deceives us (Descartes, 2008, p. 17, AT VII 25). But, according to the French philosopher, how can the evil demon deceive us? The evil demon can cause us to believe that 2+3 is more or less than 5, that the triangles have more or less than three sides, that the sky is red when in fact it is blue, that the trees are blue when in fact they are green and brown, and so on. An as examined above, even though there is a slight reason to assume that the evil demon exists and deceives us, nothing is certain until it is not shown that the deceiver cannot exist. Consequently, in this thought experiment Descartes invites the reader to suppose that there is an evil demon who is a deceiver and a liar.
As Descartes commends the reader, we should be afraid of the evil demon for this reason: "'Will you guarantee that I need have no fear or apprehension or worry about the evil demon? Even if you give me every possible reassurance I am still exceedingly afraid of coming down here" (Descartes, 2008, p. 368 AT VII 539). Then, we should be afraid of a demon who can deceive us about everything. For example, he could deceive us about our senses, about our memory, about our body, and even about mathematical and necessary truths. The evil demon is, indeed, quite dangerous from an epistemological point of view: he seems to have enough power to undermine any evidence about every belief, so no certainty is guaranteed.
One may change Descartes' though experiment slightly, though, to make the evil demon even more cunning and powerful. In fact, one may suppose that no God exists, and that the evil demon has a supreme power, one that is enough to make us believe a monstrous lie, that is, the evil demon may cause us to believe that God do exist when in fact He does not. Why would the evil demon want to cause us to believe that God exists when in fact He does not? The demon could become an infamous big liar in order to be as evilest as he could be. By causing us to believe that God exists when in fact He does not, we would be caused to believe in heaven, when there is no heaven, in benevolence and the good, when these are mere illusions. By doing so, the evil demon would accomplish the evilest task, namely, he would cause a monstrous lie: he would laugh at us by making people believe that God exists when he does not. Could the evil demon accomplish such a feat? It seems that, in so far as he is cunning and powerful, he could deceive us, even regarding the existence of God.
Perhaps a key to determine whether the feat can be accomplished by the evil demon lies in the careful examination of the third section. As analyzed, the idea of God in our intellect, the objective reality, needs to be caused by a cause that has more reality than its effect. Since the idea of God entails infinitude and perfection (among other attributes), and we human beings are finite and imperfect, the idea of God cannot have been caused by the human being. Nor can it have been caused by the evil demon. In fact, the cause of the idea must be more real than the idea itself. Therefore, only God can be the cause, the formal reality, of the idea that we have about the existence of God, the objective reality. In other words, only God, the formal reality, may have caused the idea of God, the objective reality.
Surely, a cause with less power than God cannot have produced the idea of God. But, the evil demon, although powerful and cunning, cannot be infinite and perfect. Indeed, the evil demon is mighty and powerful, but he is imperfect. How do we know this? If the demon were perfect, he would not be a liar. Indeed, lies and errors are imperfections, according to Descartes: 2+3=6 is an imperfection, like a lie, for example (Descartes, 2008, p. 258, AT VII 376). By contrast, mathematical truths and eternal truths are the creation of a perfect and supreme being. Against this background, it turns out that the evil demon cannot have us wrongly believe that God exists, when in fact he does not. In order to be the ultimate liar, the evil demon would need to be perfect, but since he is not perfect, as a liar, he cannot have caused us to believe that God exists. Therefore, there is only one lie that the demon cannot tell, viz., God exists, when He does not. In view of such imperfection, the following question arises: Can God have created evil and imperfection, if He is supreme and perfect? The last section will precisely deal with this problem, which is the mirror argument of the fourth section.

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The other side of the coin: has God caused the existence of evil and imperfection?
According to a self-evident moral principle, which is like the principle of non-contradiction, good is to be done and evil must be avoided. However, "even a perfectly virtuous person is afflicted by a proneness to evil, for which the medicine of suffering is still necessary and important" (Kretzmann;Stump, 1993, p. 263). Despite this view, the creation of evil has always been a puzzle, especially as to whether a perfect being is the primary cause of everything in the universe. It seems difficult to fathom and grasp how a perfect being could have created evil. Furthermore, it seems difficult to explain how the sin is a possibility and, even worse, how capital sins such as pride, envy, gluttony, greed, lust, sloth, and wrath exist. As I will examine in this section, it is hard to elucidate that God, as a primary cause of everything, is the creator of evil. The analysis carried out here will focus upon the mirror argument of the fourth section. Recall that there it was examined whether the evil demon has caused the idea of God. Here I will explore whether God can be the cause of evil given the conceptual apparatus of the third Meditation. 5 Traditionally, the notion of evil as a defect comes from various sources, for example, from the Neo Platonics such as Plotinus (2018, p. 11-112, I, 8,3), the Stoics such as Epictetus (1984, p. 111). All these views aim at debunking the myth that evil is as real as the good. However, the most well-known defense of this thesis is found in Aquinas, who claims that a good thing is desirable, and desirability is a consequence of perfection, because things always desire perfection. In addition, the argument for the existence of evil is as follows: every created thing is an entity, and since evil is something created, evil is an entity (Aquinas, 2003, p. 55). How does Aquinas respond to this argument? Briefly put, evil is a defect, like a limp who has a defective leg (being sane is good; having a defective leg is unhealthy). Defects are not entities. Therefore, evil is not an entity.
For the present discussion, I will mainly focus upon the reverse argument of what I analyzed in the fourth section: whether the evil demon could have caused the idea of God in our minds. Here I analyze what Descartes might have held in relation to God and evil. Apparently, God created evil, because He has created everything. This is especially important: although God is perfection, he created the human being with a will that is prone to evil, sin and imperfection. But there is a Cartesian solution to the puzzle. This passage summarizes how the problem could be tackled by the French philosopher: The more skilled the craftsman the more perfect the work produced by him; if this is so, how can anything produced by the supreme creator of all things not be complete and perfect in all respects? There is, moreover, no doubt that God could have given me a nature such that I was never mistaken; again, there is no doubt that he always wills what is best. Is it then better that I should make mistakes than that I should not do so? As I reflect on these matters more attentively, it occurs to me first of all that it is no cause for surprise if I do not understand the reasons for some of God's actions; and there is no call to doubt his existence if I happen to find that there are other instances where I do not grasp why or how certain things were made by him. For since I now know that my own nature is very weak and limited, whereas the nature of God is immense, incomprehensible and infinite, I also know without more ado that he is capable of countless things whose causes are beyond my knowledge […] It also occurs to me that whenever we are inquiring whether the works of God are perfect, we ought to look at the whole universe, not just at one created thing on its own. For what would perhaps rightly appear very imperfect if it existed on its own is quite perfect when its function as a part of the universe is considered. (Descartes, 2008, p. 38-39, AT VII 55-56).
Thus, evil is a theological problem more than a philosophical one. Still, Descartes briefly touches on the subject in a letter to Mesland in May, 1644:

I do not know that I laid it down that God always does what he knows to be the most perfect, and it
does not seem to me that a finite mind can judge of that. But I tried to solve the difficulty in question, about the cause of error, on the assumption that God had made the world most perfect, since if one makes the opposite assumption, the difficulty disappears altogether […] The moral error which occurs when we believe something false with good reason -for instance because someone of authority has told us -involves no privation provided it is affirmed only as a rule for practical action, in case where there is no moral possibility of knowing better. (Descartes, 1997, p. 232-233, AT IV 113, 115).
Descartes, then, seems to prefer to suspend judgement to avoid the puzzle of how evil exists. His attitude towards theological problems can be summarized in the view according to which God's perfection and infinitude cannot be fathomed and grasped by the human mind, because several attributes of God cannot be fully understood by imperfect beings. He declares in several passages, before and after the Meditations, that the human mind can know what God is, but cannot fathom and conceive his perfection and infinitude. Take, for example, these two passages, one from a letter to Mersenne in May, 1630: I do not conceive them [the eternal truths] as emanating from God like rays from the sun; but I know that God is the author of everything and that these truths are something and consequently that he is the author. I say that I know this, not that I conceive or grasp it; because it is possible to know that God is infinite and all powerful although our soul, being finite, cannot grasp or conceive him. In the same way we can touch a mountain with our hands, but we cannot put our arms around it as we could put them around a tree or something else not too large for them (Descartes, 1997, p. 25, AT I 151-152).
Thus, Descartes proceeds to suspend judgement on certain theological problems, and especially in those involved in comparing God's perfection and us. If asked whether God is the cause of evil, Descartes would completely avoid the problem: it would remain problematic how evil and sin exist. Oddly enough, Descartes was influenced by the Stoics, the Epicurean and Fideism concerning the role of philosophy as the pursuit of the right judgement (Rutherford, 2016, p. 2), which seems for Descartes a means to achieve happiness (Cottingham, 1998, Gueroult, 1984, Pereboom, 1994. This may explain why grasping God's perfection is a problem with which we should not deal. Evil, then, seems fundamentally related to our imperfection and, especially, to our inability to grasp perfection. However, this should not discourage the human being to find happiness. Despite the appearances and in opposition to some Christian thinkers, Descartes' ethics takes philosophy's practical goal to be the realization of a happy life: one in which a human being can hope to pursuit happiness in this life. This can be achieved by loving life without fearing death (Descartes, 1997, p. 131, AT IV 480), and by seeking virtue as a perfected power of judgement. Indeed, Descartes thinks that virtue is sufficient for happiness in the form of tranquility. Although I cannot further analyze Descartes' ethics in this essay, one thing can be said, namely, the way to achieve happiness, by practicing virtue, requires the recognition that God is the creator of all things, and that the soul is immortal. Without these two beliefs, the perfect contentment of mind (See for example Descartes, 1997, p. 256-258, AT IV 264) and the satisfaction that accompanies virtue cannot exist. The evil demon, then, represents a fundamental peril for the exercise of the free will and the soul perfection. For this imperfect being puts in danger a crucial condition for happiness and tranquility, namely, the way designed by God to master ourselves through a rational free will.

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Conclusion In this paper, I have offered an argument about the Cartesian evil demon, viz., the impossibility that he were a liar who lied about the existence of God. In order to provide a theoretical framework in which the argument made sense, I analyzed diverse topics related to Cartesian philosophy.
Firstly, I looked at how Descartes introduces the problem of the existence of God, in the Meditations on First Philosophy. In particular, I revised the form in which God guarantees knowledge certainty: if He does not exist, and there is an evil demon who deceives us about everything, no knowledge claim can be certain. Thus, according to Descartes, the existence of God needs to be proved, since no certainty is guaranteed otherwise.
Secondly, I mentioned that Descartes' offers two arguments of the existence of God, the a priori proof, and the a posteriori proof (or the cosmological argument). In view of the argument discussed in the fourth section, about the impossibility that the evil demon succeeded in deceiving us about the existence of God, I elucidated crucial notions and concepts such as objective, formal and eminent reality. All of them are fundamental to grasp how it is impossible that an imperfect and finite being, the human being for example, has caused the idea of God.
Thirdly, I dealt with the issue of how evil the evil demon can be. More precisely, I offered an argument according to which, since the evil demon is an imperfect being, he cannot be the cause, the formal reality, of the idea of God (which is an objective reality). In other words, even if the evil demon intended to deceive the human being by making them believe that God exists, when in fact He does not, the evil demon would completely fail. The rationale of the argument is that the evil demon, as a liar, cannot be a perfect being, because lies and errors are imperfections.
Finally, I considered what would occur if the argument of the fourth section were reversed. The mirror argument makes us question what would occur if God is the creator, the very cause, of evil. In particular, I gathered textual evidence to show that Descartes tends to avoid theological problems by assuming that we humans, who are finite and imperfect, cannot fully grasp what perfection really implies. That is, even though we know about the existence of God, attempting to fathom what his perfection implies is like trying to embrace a mountain with our arms. A caveat is necessary here, however: Descartes thinks that errors and lies are imperfections or defects; thus, his view seems to be akin to Plotinus and Aquinas' in relation to evil. However, I insist that the French philosopher claims that no complete understanding of divine existence can be fathomed.
The last analysis was on the evil demon and Cartesian ethics. Since Descartes' ethics holds that the goal of wisdom is happiness, and virtue requires knowledge and a rational free will, the evil demon represents a great hindrance. In so far as God did not exist, and the evil demon were a liar, there would be no room for certain knowledge and a rational free will. In other words, there would be no room for a free rational will in case we were preys of the evil demon's lies, as truth would be concealed and misrepresented by him. Therefore, truth and knowledge are crucial for Descartes, which explains why the proofs of the existence of God are so crucial in the Meditations and Principles of Philosophy. For, unquestionably, God's benevolence allows us to exercise our free rational will, the foundation stone of a virtuous, tranquil and content life.