I was thinking about Descartes’ classic example of doubt. His attempts to identify the most secure form of knowledge lead him to his own consciousness as the unassailable ground of knowing. I agree with his assessment though I think it misses an essential point, namely, that there is an existential as well as a epistemic puzzle behind the normal lack of doubt about our own existence or consciousness. Of course this is not a reason to think that consciousness and existence are domains outside the scope of doubt, but it does suggest that the lack of doubt reveals an essential aspect of how we know our own existence. I think that the essence of how we experience and know ourselves is tangential to the kind doubt that concerns Descartes, who is after certainty.
Doubt is the fundamental state of the scientist. He is always trying to reduce uncertainty to a minimum. Scientific knowing assumes a cognitive kind of doubt, as opposed to existential doubt, which is not about reducing uncertainty at all. To see this point, consider the usual kind of scientific doubt — for example I may express uncertainty about whether the sun goes around the earth or the earth goes around the sun. The evidence in front of me favors the first, but a more careful examination of the data suggests the second one. In this way, we have arrived at a less doubtful hypothesis.
Existential doubt is rather different — it takes as its focus the very possibility of something coming into existence or a perplexity about its very being what it is– it is a doubt about the essence of something. For example, I can have a deep existential doubt about the possibility of nirvana, that comes from rejecting all accounts of enlightenment as being unsatisfactory or not qualifying as true freedom. In this case, the doubt itself comes into being when the normal kinds of uncertainty are resolved — for example existential doubt entered into the Buddha-to-be’s life when he realized the absolute certainty of suffering and death.