To review one aspect of the model being explored here:
The concrete world is a causal network of events; each event is an actualized outcome, selected from a set of possibilities*. Some form of modal realism is true: while the unactualized possibilities aren’t themselves concrete, they are real in some sense. Their reality is implicated in everything that happens.
I speculate that since we’ve evolved in this kind of world, we are naturally acquainted with possibilities. In fact, the consideration of possibilities is central to life (of animals, too) and to our reasoning. Somehow, humans leverage this acquaintance with possibilities to spin whole scenarios of how a world could be.
Contemporary rationalism takes the form of modal metaphysics – where one claims that our faculty for conceiving possible worlds is reliable. Like older forms of rationalism, it is vulnerable to critics who claim we can’t know about anything which is not experienced concretely.
But on this account, the space of possibilities is involved in every concrete event. These “abstract” entities are real and are implicated in causality. Therefore our modal reasoning (and by extension, our contemplation of all sorts of abstract concepts) is not disconnected from the empirical realm. Our rational faculties are grounded in our direct acquaintance with something real.
* An alternative account would characterize each event as a manifestation, resulting from an intersection of probabilistic dispositions, or propensities. In quantum physics, the analogues are the measurement event and the wavefunction.