Like Form in Aristotle's system, it requires an object on which to lay its powers and this object is Essence. Just as form outranks Matter as the higher principle in Aristotle's thought, so does Existence outrank Essence as the higher principle in Aquinas' thought. For instance, the essence of man was that he is a rational animal. Without existence, man's essence is nothing. With existence, the principle of actuality, the essence of man completes itself.
Existence depends upon essence, but existence outranks essence.
Thus Aquinas believed in a hierarchy. He also believed that some essences possess more existence than other essences. An essence that is least contingent upon another essence for its own being has more existence. As you go up the triangle, each type of being claims a greater responsibility for its own being. On up the scale to plants, animals, and men, we see an increasing participation in the act of existing, an increasing share in the responsibility for one's own being.