Desperate Measures
I am going to pose myself a challenge.

In the constant quest of arguing once existence, there is a worrying problem that pops up after a while that for I cannot get my head around at all.

Say that my existence is the thought of some entity, simply a short-lived idea, maybe only lasting a few seconds as it scutters around the hallways of the mind. If this is the case, then how can I argue, or explain that my existence is something other than the imagination of someone or something. Descartes would explain this away with his axiomatic Cogito ergo sum, but this relies on the assumption that the action of thinking is incorruptible in itself, not subject to anything, and only the primary external senses and ideas derived from these are under the power of some demonic entity out to get you. The problem which presents itself, however, is that nothing is inviolate if this situation is as presented above. Every logic one can come up with is corrupt, and so is every fact, experience and notion that might present itself in this existence. Though these might be objective to oneself and current existence, they are subjective and nested within it and unapplicable if one's entire existence is put to the question. Therefore, they cannot be used to solved the problem. It's an impossible, and infuriating loop of thought.

The very action of thinking might simply be imagined in itself, and though you could say that because I imagine, therefore I am, it is not my imagination that brings everything forth, it is that of the entity doing the imagining. Not me. So though I cannot doubt existence in general, mine can with ease be put to the question.