The Difference Between Essence and Existence

You can know a thing's essence without knowing whether or not it exists. The biological facts about lions and pterodactyls would be exactly the same whether or not we were around to study them. And if a thing exists, then its existence too is obviously part of its objective reality. So this argument under Roman numeral I fails for there is a crucial disanalogy between what is uncontroversially a part of athing's essence on the one hand and the existence of the thing on the other.