Existence (n.) is not necessary, but if I am thinking then I, necessarily, exist.
Existence (n.) is the existence of the ego or subject. Whatever else exist exist because it is represented in it.
Existence (n.) is proved by the act of thinking — Cogito ergo sum. I think therefore I am.
Existence (n.) is the one thing you cannot doubt while doubting: you may suppose away God, sky, hands and feet, but not that you are while you doubt.
Existence (n.) is proved by being deceived: the deceiver can never make me nothing so long as I am conscious that I am something.
Existence (n.) is a perfection, so a supremely perfect being lacking it is a contradiction.