The Ablative Case is historically a conflation of three other cases: the true ablative or case of separation ("from"); the associative-instrumental case ("with" and "by"); and the locative case ("in"). The process of conflation has meant that originally distinct meanings have become merged and we do not always know whether a particular usage belongs to one or the other of these categories. The ablative of cause is a good example of how the different origins of the ablative case can reinforce each other. Many instances of the ablative of cause may be analyzed in two ways: e.g., vulnere mortuus est. This could be understood as "he died from a wound" (the original case of separation) or "he died by means of a wound" (the original associative-instrumental case). In fact, one sometimes finds ex vulnere mortuus est which is a clear indication that the Romans felt that "origin" and "cause" were related. Caesare duce: Similarly, this kind of ablative absolute may be analyzed as a temporal ablative (that is, a metaphorical extension of the locative case) or as an associative-instrumental ablative (for instance, in the sense of "with Caesar being the leader"). On the other hand, some uses of the ablative are precise and involve category distinctions that should not be overlooked. EXAMPLES: Caesar militibus urbam oppugnavit = "Caesar attacked the city by means of his soldiers", that is, the soldiers were his tools, the means by which he carried on his assault. Caesar cum militibus urbem oppugnavit = "Caesar attacked the city with his soldiers"; that is, he was accompanied by his soldiers in the attack. A militibus urbs oppugnata est = "The city was attacked by the soldiers"; that is, by the soldiers as volitional agents.
The True Ablative (Ablative of Separation)
All Ablatives after the prepositions ab, de, and ex are originally Ablatives of Separation. Similarly, the Ablative after a verb or separation, freeing, difference, and movement away from is also an Ablative of Separation: e.g.: metu relavatus = "relieved of [separated from] fear." Some idiomatic usages are usefully distinguished:
The Associative-Instrumental Ablative ("with")
The Ablative with or without the preposition cum can indicate a person, thing, or quality associated with the activity of a verb. With the preposition, the meaning is usually apparent from a simple translation of the preposition. The most usefully distinguished types of associative-instrumental ablative are:
This usage has a peculiarity in that, when the noun is modified by an adjective, you do not need to use the preposition: magna [cum] laude = "with great praise"; magna celeritate = "very quickly".
Degree of Difference: The Ablative of Degree of difference is a development of the ablative of means: the amount of the difference being considered the means by which something is different. Consider: a wall higher by ten feet.
The Locative Ablative and Ablative of Time
The ablative after prepositions of place or time denotes location in place and time. This is to be distinguished from the accusative after the same preposition which indicates motion into, down under, toward, etc. Place: the preposition is omitted with the names of cities, towns and small islands, with a few idiomatic expressions (like terra marique, loco, regione, parte, etc.), and frequently when a noun is qualified by adjectives denoting some part of the whole: summus, imus, medius, totus, omnis, cunctus, universus. Time: the ablative of time is used to indicate 1) a point in time at which something happens, 2) a period of time during which something happens: this is similar to the accusative case and is found more frequently with negative verbs (it did not happen within this time span) than with positive verbs (it happened during this time span). Although the accusative's sense that something happens during is different from the ablative's meaning that something happens within, still one finds examples in Classical Latin of some confusion between the two: tota nocte continenter ierunt For five years in the African way, and then for nine years in Spain he acted that way. "
The Ablative of Respect seems to have had a composite origin or was influenced at least by two different case-functions: the locative function and the instrumental function. An example that is easily assimilated to the locative function is the following: non tota re sed temporibus errasti (Cic. Phil. 2. 23) = "you were wrong not in every regard (in every thing) but in respect of you dates." On the other hand, the instrumental function seems more clearly at work in expressions like: uno oculo captus = "blind in one eye" or Cicero nomine = "Cicero by name."
The Supine in the Ablative is often identified as an Ablative of Respect: mirabile dictu = "marvelous in respect of saying"; facile factu = "easy in respect of doing". This usage is thought to arise from the Ablative of origin and that view is supported by the Supine of Source found in early authors: e.g., cubitu surgere (Cato RR 5.5) = "to arise from sleeping." However, it is clear that by the Classical period these supines had a clear affinity for the locative function of the ablative.