existence...

Like many philosophically interesting notions, existence is at once familiar and rather elusive. Although we have no more trouble with using the verb ‘exists’ than with the two-times table, there is more than a little difficulty in saying just what existence is. Existing seems to be at least as mundane as walking or being hungry. Yet, when we say ‘Tom is hungry’ or ‘Tom is walking’, it may be news to those not in Tom's vicinity, whereas ‘Tom exists’ would be news to no one who knew Tom, and merely puzzling to anyone who did not. Again, we know what it is like to be hungry or to walk, but what is it like to exist, what kind of experience is that? Is it perhaps the experience of being oneself, of being identical with oneself? Yet again, we can readily indicate what is meant by Tom's walking, but surely Tom's existing is not something we can indicate to anyone. On the face of it, there would seem to be no way at all in which we can explain what existing is. [Emphasis mine.] Stanford Encyclopedia of Philosophy: Existence

but, who is Tom...?