Is there anything that is beyond question and that which is beyond doubt? (as everything is temporal, including the subject)
An obvious answer to this is the existence of the subject, i.e., the self that considers the question. This is partially what the famous Cartesian assertion, cogito ergo sum ("I think therefore I am") refers to.
There are ways to nit-pick Descartes' formulation, however. It does seem syllogistic, i.e., actually based on two premises:
- That thinking things exist.
- That I think.
One problem this exposes is the issue of who or what it is that actually thinks; "I think therefore..." takes as given that it is in fact the I subject which thinks. Yet conjecture could lead us to the conclusion that it might be something else which thinks, and the I is simply an observer of this with itself as the subject.
A refinement would be to ditch the the "think" but keep the "I" and "exists", such that the subject is proof of existence. So there are two unquestionable things, the subject, and existence. This implies it might be possible for there to be existence, but no subject, in which case existence is the fundamental ground of the subject. Solipsism is a challenge to this, asserting that without the subject there might be no existence, in which case existence and the subject are one and the same.
everything is temporal, including the subject
This is not true; some facts about existence could be said to be non-temporal even if they are not certain. For example, that 1 + 1 = 2 is a belief, but if it were true, then it does not depend on a thinking subject, or time, to be so.
Time is meaningful only in terms of relation, which is erased by the solipsitic challenge. A solipsistic subject might pragmatically deploy the concept of time (and "existence") in self-reflection, but this does not make it otherwise real.