Aristotle’s notion of “being” has two poles. On the one hand:
“Substance [or "being" - ed.] is thought to belong most obviously to bodies; and so we say that not only animals and plants and their parts are substances, but also natural bodies such as fire and water and earth and everything of the sort, and all things that are either parts of these or composed of these (either of parts or of the whole bodies), e.g. the physical universe and its parts, stars and moon and sun. But whether these alone are substances, or there are also others, or only some of these, or others as well, or none of these but only some other things, are substances, must be considered.
Metaphysics bk. VII c. 2
What is physical, concrete, sensible, extended, and treated by physical science is the first thing meant by a being. Any other being is only an inference from these, and one is not to give the benefit of the doubt to any other kind of existence. But after developing this notion of being, he finally concludes that:
Evidently even of the things that are thought to be substances, most are only potencies,-both the parts of animals (for none of them exists separately; and when they are separated, then too they exist, all of them, merely as matter) and earth and fire and air; for none of them is a unity, but as it were a mere heap, till they are worked up and some unity is made out of them.
Metaphysics bk. VII c. 16
The passages are pretty clearly parallel, and the tension between them explains a good deal about what makes metaphysics difficult. What has parts (what is extended spatially) is the foundation of our idea of being, but we are capable of figuring out that it barely meets the criterion we have for what is and exists. In diverse ways, the spatially extended verifies our notion of being most of all and least of all. This imposes on us the need to figure out a way of speaking about being that can preserve the truth of this. St. Thomas’s division of analogues into those that are first known and those which most verify that which is known (the mode of signification vs. the thing signified) is a structure that is hard to improve upon.