For a chair to exist is simply for a particular bunch of particles, or maybe larger chunks like pieces of wood, to be arranged chairwise (which may refer both to physical arrangement and to the purpose). To put it in Thomistic terms, a chair's esse, its to be, is the arrangement of a bunch of things. Notice that the esse of a chair, then, does not mention the chair—for a chair to exist is for some other things to have a particular arrangement. The chair, thus, is a second class citizen of Thomistic ontology: claims of the existence of the chair can be reduced to claims about the arrangements of particles or chunks. Chairs are not substances.
What is the esse of a contingent substance? Aquinas does not tell us as far as I know. But we can speculate that the esse of a substance is one of the following:
- participating in God's esse
- being caused
- being caused by complex of causes C.