"There is not one of my former beliefs about which a doubt may not properly be raised"(Descartes).
Does Descartes succeed in showing this in the First Meditation?

The answer is a qualified "Yes!"

Descartes was concerned about the proper basis for "certain knowledge".   Like his Scholastic predecessors, he thought of "knowledge"as something that was absolutely certain.   Where there is no certainty, there is no knowledge.   He was, moreover, aware that there were many things that he thought he "knew"that were based on highly dubitable foundations.   So in an effort to establish a properly certain foundation for "knowledge", he proposed to systematically examine all of his beliefs, and reject as "knowledge"any of which he could find that were not certain.

To do this, he adopted what has come to be called "methodological doubt". He examined the basis for entire classes of beliefs, and recognized in each case, that there was some reason to doubt the certainty of the basis.  

Now Descartes recognized three fundamentally different foundations or origins for his beliefs.   Some of his beliefs were the result of the impressions of his senses.   Some were the result of the impressions of his memory.   And some were the result of relationships between ideas in his mind.   Because of his Mind-Body dualism, Descartes treated the latter category of ideas differently from the first two categories.   More on the consequences of that in a moment.

In order to demonstrate that any of his beliefs that originated in the senses or in the memory were dubitable, Descartes employed three different arguments -- the argument from error, the argument from dreams, and the argument from the evil deceiver.

In the argument from error, Descartes reasoned that any of his beliefs formed on the basis of his sensory perceptions could be doubted because sensory perceptions can be easily shown to deliver apparently erroneous information.   The standard current example of this phenomenon is the straw in the glass of water looking bent when seen from an angle.   In a similar fashion, he argued that he had upon occasion remembered things incorrectly.   Hence, his conclusion that any of his beliefs that originate in sensory impressions or in memory must be considered dubitable.   He has no way of telling, for any given instance, whether his sensory impressions are erroneous or not, or whether any given memory is correctly recalled or not.

In Meditation II, Descartes reinforces this argument by using the example of the lump of wax to demonstrate that the wax can remain the same lump of wax while undergoing complete reversals of sensed properties.   The wax is both hard and soft, fragrant and not, of a solid mass and a shapeless liquid, and so forth.   Since the senses obviously cannot be relied upon to provide the "proper"knowledge of the lump of wax, any beliefs about the nature of the wax that result from sensory impressions cannot be relied upon to provide indubitable knowledge of the wax.  

In the argument from dreams, Descartes reasons that when he is dreaming, he seems to sense and remember things as if they were real.   Yet, ex hypothesi, they are not real because he is dreaming.   And he cannot tell, when he is dreaming, that he is dreaming.   So it is possible that everything he is now experiencing and remembering is but a dream.   Hence, none of the beliefs that he now has about what he is sensing or remembering is indubitable.   They could all be the result of a dream.

Finally, and most famously, Descartes reasons that the gods, being all powerful, are powerful enough to be deceiving him about all that he perceives and remembers.   This is Descartes'famous "Evil Deceiver"or "Evil Demon".   His "Evil Demon"hypothesis is based on his belief in the fundamental separation of mind and body.   Descartes'theory of sensory perception separates the act of sensory perception (that he believes takes place in the body) from the understanding of that perception (that he believes takes place in the mind).   In his theory, the senses (and memory) "present", in some unspecified fashion, the relevant impressions in what has come to be called the "Cartesian Theatre"where the Mind watches and interprets.   Given such a separation, there is an opening between the "presentation"and the "interpretation"where the Evil Deceiver can come into play.   This allows Descartes to posit that what his Mind is watching in the Cartesian Theatre is not the reports of his senses and memory, but the deceitful images provided by the Evil Deceiver.   And all of the perceptions that lead him to believe in the existence of his body are but the result of those false impressions provided by the Evil Deceiver.   Perhaps, Descartes reasoned, he is only a mind, and not a body.   (The modern sci-fi variation of Descartes argument is the "Brain-in-a-Vat"scenario.)

By these three lines of argument, Descartes has successfully raised doubts about the veracity of the information provided by his senses and his memory.   None of the inputs that cross the Body-Mind divide can be relied upon as the basis for certain knowledge.   Since Descartes is, at this point in his meditations, of the opinion that all of his beliefs are based on the information provided by the senses or memory, he can fairly conclude - "There is not one of my former beliefs about which a doubt may not properly be raised."So in that sense, he has been successful in challenging the certainty of those kinds of beliefs.

However, as I noted above, Descartes recognized three different foundations or origins for his beliefs.   He has successfully demonstrated that doubts may be properly raised about two of those foundations.   But he does not thoroughly challenge those ideas that are based on the relation of ideas already in his mind.   Although he allows that the Evil Deceiver might deceive him about such "exceedingly simple"ideas as that two plus three is five, he does not permit the Evil Deceiver to deceive him about his ability to reason.   Descartes therefore claims that his beliefs that are "manifest by the natural light"(or as we would now call them "logically necessary truths") are indubitable and not open to deceit by the Evil Deceiver.

The first of these is, of course, that his reasoning capabilities remain unsullied by deceit.   So Descartes can be comfortably certain that "[I]t must, in fine, be maintained, all things being maturely and carefully considered, that this proposition 'I am, I exist', is necessarily true each time it is expressed by me, or conceived in my mind."[Meditation II-3]   The second of these is that Body and Mind are distinct substances -- granting an opening for the Evil Deceiver.   Descartes does not challenge any of the Scholastic traditions that he takes as "intuitively obvious".   Like the key premise of his proof of the existence of God, demonstrated by Darwin to be quite false -

Now, it is manifest by the natural light that there must at least be as much reality in the efficient and total cause as in its effect; for whence can the effect draw its reality if not from its cause ? And how could the cause communicate to it this reality unless it possessed it in itself? And hence it follows, not only that what is cannot be produced by what is not, but likewise that the more perfect, in other words, that which contains in itself more reality, cannot be the effect of the less perfect; and this is not only evidently true of those effects, whose reality is actual or formal, but likewise of ideas, whose reality is only considered as objective.     Descartes, Meditation III

With the exception of his presupposition of Body-Mind duality, most of his lapses in methodical doubt appear in the Meditations after his general conclusion (quoted in the title) at the end of Meditation I.   So if the question is focused on whether Descartes has successfully shown -- by the end of Meditation I - that all of his sources of belief are not indubitable, then the answer must remain a qualified "Yes!"His Evil Deceiver hypothesis represents an insurmountable doubt in the certitude of any beliefs at all.   As long, that is, as he lets the Deceiver play the role Descartes assigns to him in Meditation I.   And even though the very possibility of the Evil Deceiver is founded on an unchallenged belief in the separation of Mind and Body.

On the other hand, if the if question is broadened somewhat to include within its scope the limitations that Descartes places on the role of his Evil Deceiver in Meditations II and III, then the answer has to be "No!".   Descartes very naturally excludes from the domain of the Evil Deceiver his belief in the very reasoning abilities that he relies upon to progress his arguments.   But he also excludes from this methodical doubt the key scholastic premises that he employs in his analysis of ideas, and his proof of the existence of God, in the remainder of the Meditations.   So, ultimately, Descartes cannot be said to have shown that "There is not one of my former beliefs about which a doubt may not properly be raised."Contrary to his claim, Descartes very clearly reserves some of his beliefs as beyond doubt.

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