As Descartes realized, I think, therefore I am. As Hume realized, beyond this, I know nothing.
For contrary to Kant and later Hegel, the existence of A does not imply the existence of not A. A is a symbol, representing a concept of an underlying reality which I do not know to exist. If the underlying reality of A is unknown, so too is the underlying reality of not A. Therefore, I do not know that anything besides me exists, regardless of my imaginings. Certainly I cannot know, as Kant and Descartes hoped, that the existence of me or my imperfection implies the existence of any deity or of any perfection.
That I have symbols, that I imagine does not mean that anything exists apart from me, for it may all be the construction of a greater me, temporarily lost in imagination. It may be that I am a masochist in a void, or in nothing else at all, projecting a hellish existence, so like someone suffering depression, I will feel pain that I may feel anything at all. I do not know. I cannot know.
I know that I perceive an often hellish world, an existence of all the cruelty I have witnessed and in some way experienced. I do not know whether or not this world, this hell in fact exists. I do not know that the people I perceive in it exist. But I know I want contact with some people I perceive and even if I have imagined up this world, I live in it, even if I can never be certain of its existence.
Post-modernists submit that if nothing beyond perception can be known, then the only proper field for investigation is within the self, that we can write not of things outside ourselves but only our experience of those things. But to deny the objective is to deny that which we all, assuming that more than one of us exist, have in common. We may not perceive the objective accurately. As Hume observed, there is no necessary correspondence between what we perceive and what is real at all. But to discount it entirely is to devalue any communication, or more cynically, to accumulate publication credits toward tenure at a university with the contents of my toilet.
If I am to communicate with you, I must assume that you exist and that we have in common some means for our communication. We must strive for that which we have in common, the objective which we may poorly or not at all perceive, even if we can never succeed. And so it is on this basis I proceed, not knowing the correspondence (if any) between my perception and reality, and by rejecting post-modernism for its denial of an objective that I must fervently wish to exist.