The theoretical intellect strives toward truth and understanding. The practical intellect deliberates about what is the best course of action. It begins with inclinations provided by the appetites, but corrects and supplements them from a rational assessment of a plan of life. One attempts to achieve one's ends by choosing appropriate means. Essences provide a non-arbitrary principle for dividing the world into distinct, countable individuals.
Essences provide a non-arbitrary principle for identity through time.
The dog's function is to be loyal and trainable, its use is to herd sheep. For example, the object of vision is a scene made up of shapes and colors, the object of hunger is food, the object of thought is the truth. The members of a species share exactly the same nature or essence. Each thing in nature belongs to exactly one species. Truth itself is one, indivisible.