We differentiate between existence and the concept of existence. If you want to directly perceive existence, simply look around you. If you want to grasp the concept of existence you break down, or reduce the concept to identify what has been mentally integrated. You break down what units were isolated by the process of abstraction. You break down what the abstraction was isolated from.
If you check out this piece, one of the characteristics or properties of every entity mentioned, is that it exists. You can further identify that if a rock was thrown, it moves through the air. The motion exists. When you weigh the rock, it’s specific weight exists, and so on.
Existence exists integrates all these facts together. The rock exists, Its weight exits. Its mass exists. A blade of grass exists. Its length, width and breadth exist. Existence exists integrates these into a single propositional principle. It, as well as the law of identity and consciousness, are axioms, because they are implicitly contained in every possible identification.