Originally an Ablative of Separation. This Ablative is used most often with prepositions meaning "from" (de, ex, ab) and with certain expressions of origin or birth, like natus deo = "born from a god." The Ablative of Separation is a metaphorical use of the idea of separtion: it imagines the entity to which another entity is compared as a kind of standard from which the thing-compared "departs." To tediously translate the metaphor, one might say something like "Moving from the standard set by Balbus, Marcus is stupider." That would be: Balbo Marcus stultior est. or "From Balbus, Marcus is stupider." In fact, even in Augustan Latin the basic sense of a Standard of Comparison from which something departed was so strong that a new idiom develops: a Venere non est pulchrior ulla = "from Venus none is more lovely" or "No one is more lovely than Venus."