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Course Hero. "Meditations on First Philosophy (with Objections and Replies) Study Guide." November 10, 2017. Accessed September 25, 2018. https://www.coursehero.com/lit/Meditations-on-First-Philosophy-with-Objections-and-Replies/.
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Descartes solicited objections from a number of sources. Not all of them are included in standard editions of the Meditations as commentators focus on specific themes, as is done here. Alternatively, various commentators find some of the objections to be philosophically unhelpful or redundant.
In the Second Objections, Marin Mersenne points out that the criterion for certainty itself requires explanation. By itself it is not sufficient to show that Descartes himself is not the cause of his own deception. He may well be unaware of it. After all, many people believe they have clear and distinct ideas but, unbeknownst to them, are deceived in some way.
Descartes's reply is that, if people are so convinced of the veracity of an idea—as they are when they hit upon one that is clear and distinct—then there are no further questions to ask. Furthermore, Descartes claims anyone who uses their intellect will not succumb to the sort of deception to which Mersenne refers.
Descartes refers to the sole use of the intellect to derive clarity rather than deriving "such clarity from the senses or from some false preconceived opinion." The latter will make ideas opaque, whereas the mind is transparent to itself—there is nowhere, for example, for deception to hide.