St. Thomas Aquinas viewed reality as something between material experience and God; material experience is "potency," or the potential, while God is "act," or the existing. Man has existence through Potency, because he can grow and accomplish, and through his God-given intellect he can recognize the Act. God has existence entirely through Act, because there is no place to grow further; Man cannot achieve complete Act but can strive towards it through Potency.
Another way of describing it is to differentiate between the "essence" of an object, and the "existence" of the same object. The "essence" is the thing itself, what the thing "is" without any qualifiers. The "existence" is the Act of being, the actual physical reality of an object and its position in the world. The essence of a tree is that it is a tree, while the existence of a tree is that it roots in the ground, grows towards the sky, and converts sunlight through its leaves. The tree exists through its Potency, and "is" because of its Act.
Man cannot transcend the boundaries between Potency and Act, but he can strive to understand and accept them as parts of a whole. Man also cannot become pure Act, unless the Soul is accepted to be Act housed in Potency.