 Variants of the PSR may be generated not only by placing restrictions on the relata at stake (both the explananda and the explanantia), but also on the notion of the relation at stake. Frequently, the relation of providing a reason is conceived as irreflexive, antisymmetric and transitive, though each of these characteristics may be, and indeed have been, challenged. The relation of providing a reason can be conceived as an ontological relation (as in contemporary discussions of ground), or as a purely epistemological relation.