How Does a Genus Multiply?

"We asked brother Thomas for an intro to metaphysics. We still need an argument for the distinction of essence and existence," he says. "If pure essay is unique, then there isn't anything which isn't something whose essence is its existence." He goes on to explain how a species can be multiplied in distinct chunks of matter but that doesn't apply if it's pure existence. 'It exists in virtue of itself so it can't subsist in anything other than itself'