The philosopher, Rene Descartes, has said that it is possible to doubt all
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 this proposition would make his conclusion a tautology; or it does not follow from 'There is a thought now.' Lichtenberg writes in The Waste Books, K18, that 'We should say it thinks, just as we say it lightens. To say cogito is already to say too much as soon as we translate it I think. To assume, to postulate the I is a practical requirement.' I also think you should take care to separate the questions whether Descartes proof is any good from the question whether the existence of the self is certain or can be doubted. So the phrase "otherwise stated" in your last sentence is not right. The two questions are distinct. Similarly, it is not the question whether the "cogito ergo sum" is indubitable, but whether the existence of the self is.