Spinoza's first axiom in The Ethics goes "All things which are either are in themselves or in another." Omnia quae sunt vel in se vel in alio sunt. The meaning of this, as many commentators have pointed out isn't exactly clear. But, later on, in his third demonstration of proposition 11, Spinoza seems to imply existence as power. "An ability to not exist is impotence, and contraiwise an ability to exist is power." Posse non existere impotentia est et contra posse existere potentia est. So, does Spinoza's first axiom mean that all things which exist either draw their power from themselves or from something else? What does Spinoza's first axiom mean?
Let me preface this by saying that I am not at all a Spinoza scholar, and stand to be corrected on this.
My reading is that existence is certainly a power, so that the first Axiom means (or implies) that all existent things either exist by virtue of themselves (i.e., are self-caused primitives) or by virtue of another (where "exist by virtue of" can be read as "draw their power of existence from").
This is a fairly standard place for philosophical texts to begin; about 1500 years before Spinoza, and a continent away, Nāgārjuna began his Mūlamadhyamakakārikā with the verse: "Neither from itself, nor from another, nor from both, nor without a cause, does anything whatever, anywhere arise." Among the views he was rejecting in that list is Spinoza's axiom, which was already considered classical in 2nd century India.