For every relationship involves two related terms. Sometimes relationships are not real in either term, but arise from the way we think of the terms: we think identity, for example, by thinking one thing twice over and relating it to itself; and occasionally we relate what exists to what does not exist, or generate purely logical relations like that of genus to species. Sometimes relationships are real in both terms: grounded in the quantity of both, in the case of relationships like big/small or double/half, or in their activity and passivity, in the case of causal relationships, like mover-moved and father/son. Sometimes relationships are real in only one of the terms, with the other merely thought of as related [reciprocally] to that one; and this happens whenever the two terms exist at different levels. Thus seeing and understanding really relates us to things, but being seen and understood by us is not something real in the things; and similarly a pillar to the right of us does not itself have a left and a right.
Thomas Aquinas, Summa Theologiae: A Concise Translation
logic, ontology, philosophy