“An internal relation is one that is necessary and unchanging because of intrinsic features (nonrelational features) of the relata [the things standing in relation]. The relation of ‘being darker than’ is an internal relation between blue and yellow because of the intrinsic characters of the colors blue and yellow. But ‘being darker than’ can be an external relation, for example, between my coat and my hat. That relation, depending on what colors they happen to be (imagine they are both blue), is not necessary and can change for many and varied reasons, for example, if the darker of the relata is washed repeatedly.”
— Steven Scott Aspenson, The Philosopher’s Tool Kit, 21.