Doubt (n.) is something which arises so it is utterly incoherent to attempt to feign or create doubt for the purpose of conducting philosophical inquiry.
Doubt (n.) is essential in a search after truth — If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.
Doubt (n.) things should be treated as false, so that what is certain stands out more clearly.
Doubt (n.) is a mode of willing, not of perceiving — like desiring, refusing, affirming and denying.
Doubt (n.) pushed to the limit once makes it impossible ever to doubt again wherever truth is later found.
Doubt (n.) is falling suddenly into deep water: you can neither touch the bottom nor swim on the surface.
Doubt (n.) of the earlier days, the dream doubt above all, turns out at the end to be hyperbolical and ridiculous.