Any conclusion at which one arrives after a long process of reasoning will have a possibility of error, just as I said about mathematical arguments. Consequently if we want to find some absolute subjective certainty, it will need to be something either without a process, or with a minimal one; something very basic.
First principles such as the principle of non-contradiction and the like are one possibility.
Another would be immediate apprehensions such as “I exist”, “I am thinking”, “I am awake”, and so on.
Still another would be facts of immediate experience such as “I am currently working on a blog post.”
We might come to a different result depending on whether or not we include the content of the assertion as well as the mode of apprehension. If the mode of apprehension is considered alone, “I exist,” cannot be said to have such absolute certitude if it is possible for someone to be certain in exactly the same way of the claim, “I do not exist,” and it seems to me that this might be possible. This is even more likely to be possible in the case of the claim about being awake; a waking person can believe that he is asleep and dreaming.
If we include the content of the assertion, this kind of certitude is not fully distinct from objective certitude, and there will surely be at least a few cases such as the claim about existence. Thus someone who believes that he exists, cannot fail to exist, not so much because of the certainty of his belief, but because having a belief includes existing.
External facts of immediate experience, such as the fact that I am currently writing this blog post, or that there is a computer on the table in front of me, are not infallible even including both the content and the condition of my certainty, since we are often wrong about such things, e.g. “I know my glasses were there on the table, I just saw them,” but they were not.
Overall the situation is not entirely clear. Absolute subjective certainty, including the content of the assertion as well as the mode, is possible, as in the case of the claim about existence, but it is not clear that it is possible if the content is not included. In any case, whether or not the content is included, such certainty could only refer to immediate apprehensions, and at most only to a few such apprehensions.
One other case which is sometimes proposed is the case of faith in divine revelation. Thus the Catechism of the Catholic Church states, “Faith is certain. It is more certain than all human knowledge because it is founded on the very word of God who cannot lie.” It seems likely that in reality this refers to objective certitude: one who believes something revealed by God cannot be wrong, because God does not reveal things that are false. Such an objective certitude would not imply absolute subjective certitude, for the same reasons that mathematical demonstration does not imply such a subjective certitude. However, some believe that the Catechism refers to an absolute subjective certitude. I will consider this idea in another post.