Online research in philosophy

Here's an old question in the philosophy of perception: here I am, looking at this pen [I hold up a pen in my hand]. Presumably I really am seeing this pen. Even so, I could be having an experience just like the one I am having without anything being there. So how can the experience I am having really involve direct awareness of the pen? It seems as though the presence of the pen is inessential to the way the experience is.