A thing is human only if it is mortal. Only mortals are humans. What we are saying is that these two classes do not share any common members. No trees are animals. Nothing that is a tree is an animal.
All trees are nonanimals.
If anything is a tree, then it is not an animal. A thing is a tree only if it is not an animal. Nothing is a tree unless it is not an animal. Now, we will turn to the other quantifier, the existential quantifier. The existential quantifier is represented with a backwards E and variable.