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The philosopher, Rene Descartes, has said that it is possible to doubt all things except the existence of oneself (cogito ergo sum); that it cannot be doubted, despite how hard one endeavors. However, I am often questioning if that proposition is "truly" "indubitable". I desire to know if there have ever been any well-known or ancient philosophers who had not "concurred" with Rene Descartes regarding the cogito ergo sum; or if there are modern philosophers with great reputation, prestige, or respect within the philosophical community, who believe that the cogito ergo sum is "not" indubitable? Otherwise stated, it is "possible" to "doubt" the existence of oneself.

There are plenty of philosophers who have not agreed with Descartes' line of thought here, though they are not "ancient" philosophers, as Descartes did not propound the "proof", if that is what it is, until 1637, in the Discourse on the Method and, in a slightly different form, in 1641 in the Meditations . You can find some interesting material in the "Objections" to the Meditations , for example the Fifth, by Gassendi, or the Fourth, by Arnauld. Hobbes too, in the Second Objection, makes of Descartes' argument a triviality. How (he asks) can I know that I am thinking? 'It can only be from our ability to conceive an act without its subject. We cannot conceive of jumping without a jumper . . .'. Through the centuries Descartes' dictum has come under even more fire from different directions, for example from A.J. Ayer in Chapter 2 of Language, Truth and Logic . Descartes was only entitled to say that 'There is a thought now,' not 'I think', i.e. 'There is an I and it thinks', because...

Does certainty suggest or indicate truth?

Descartes sought certainty because he thought that if we know something with certainty, then it must be true. And he was right, if only because 'S knows that p ' implies p , so that in 'We know with certainty that . . .' the phrase "with certainty" is redundant; there is no such thing as uncertain knowledge. I suspect that the sense of your question may be Cartesian: is it the case that certainty implies truth? There are several concepts to sort out here: 'We know for sure, or for certain, or with certainty that . . .', 'I am certain (sure) that . . .', 'I feel certain, sure, that . . .', 'It is certain that . . .' (but not 'It is sure that . . .') There is a very useful paper by G.E. Moore called "Certainty" that might be helpful here, which is sensitive to distinctions of this kind. Sean is right in his response above that psychological certainty or "feeling certain" may not be a mark of truth, though I wonder whether anyone has troubled to test the correlation empirically in...

How does our approach to knowledge about the past differ from our approach to knowledge about the future, keeping in mind that there is an element of uncertainty in both?

Our knowledge of the past derives from perception, memory and inference, in the sense that these are answers to the question, 'How or by what means do you know?' (There are other ways, for example report or testimony). But our knowledge of the future has in it no elements of memory or perception. So as one might therefore expect it is harder to come by knowledge of the future, and we have less of it per hour, if you want. We typically can know more about a past hour than about a future hour, though by no means all of the past hours, for example those in past centuries. If I know p, and p is a proposition about the future, I cannot know it by memory, special cases apart. (A special case would be that I come to know that I am going to Africa next summer - a piece of knowledge about the future - by remembering that I am going to Africa next summer. 'How do you know?' 'I just remembered it . . .' makes sense as a conversation.) It seems to me, in spite of the assumption you make, however, that in some...