If an entity is made out of smaller constituents, it needs cooperation in order to be selfish.

My formulation is just as correct as the standard economics one, but not as misleading, because it rightly implies that you are part of/made of larger and smaller entities, whereas the economics formulation falsely implies that you are an independent entity, capable of pure selfishness. If one cell in your body is selfish, that's called cancer and you die.