In the construction "magno argumento esse", does "argumento" take an explanatory infinitive?

Question

Cicero, Phil. 2.16: Quod autem idem maestitiam meam reprehendit, idem iocum, magno argumento est me in utroque fuisse moderatum.

Cicero, ND 1.1 (LCL 268): De qua tam variae sunt doctissimorum hominum tamque discrepantes sententiae, ut magno argumento esse debeat causam et principium philosophiae esse inscientiam, prudenterque Academicos a rebus incertis adsensionem cohibuisse.

In these two passages from Cicero, the clausal kernel is [x] magno argumento est [infinitive clause].

In the passage from Philippics, a quod clause serves as the subject. In the passage from De Natura Deorum, there is no explicit subject, but I believe the previous statement is meant to be the implicit subject.

I understand magno argumento as a dative of service argument, which is common with linking verbs.

But both passages then include an infinitive clause. The clause seems to contain the proposition for which the subject constitutes an argument.

My question is how to understand the syntax of this infinitive clause. Is it a substantive clause depending on argumento? The presence of a dative argument makes me disinclined to identify it as a predicate of esse.

Answer 1

Roby (1892: A Grammar of the Latin Language from Plautus to Suetonius, Part II, Book IV; pref. p. 30) put forward a nice parallelism between the two examples below (like yours both are from Cicero), which can be taken as evidence for the predicative nature of both argumentum and argumento (NB: due to this kind of parallellism the latter is often called a "predicative dative": e.g. see this video). Both examples also contain (i) an infinitival clause, which, as you point out, can be said to be an argument of the predicative noun, and (ii) a substantive clause introduced by the conj. quod, which is the subject. Like Cerberus I give a positive answer to the question in the title but I would like to stress that the same answer holds for argumentum below, which can also be said to take an explanatory infinitive. Your question is then independent from the fact that the predicate is a dative (argumento) since one could raise the very same question for nom. argumentum in the first example below. I'm also saying this because of your last comment in your post above ("The presence of a dative argument makes me disinclined to identify it as a predicate of esse"). By the way, why do you call the dative an "argument" here? As noted, it's a predicate.

Maxumum vero argumentum est naturam ipsam de inmortalitate animorum tacitam iudicare, quod omnibus curae sunt, et maxumae quidem, quae post mortem futura sint. (Cic. Tusc. 1. 31).

'But the strongest proof that nature itself tacitly pronounces in favor of the immortality of the souls is that everyone feels the concern, and even the greatest concern, of what must happen after death.'

magnoque esse argumento homines scire pleraque ante quam nati sint, quod iam pueri, cum artis difficilis discant, ita celeriter res innumerabilis arripiant, ut eas non tum primum accipere videantur, sed reminisci et recordari. haec Platonis fere. (Cic. Sen. 78)

'And a strong argument that men's knowledge of numerous things antedates their birth is the fact that mere children, in studying difficult subjects, so quickly lay hold upon innumerable things that they seem not to be then learning them for the first time, but to be recalling and remembering them. This, in substance, is Plato's teaching'. (Transl. by William Armistead Falconer, 1923, Perseus site)

Answer 2

Your analysis seems to be correct. Argumento (esse) is like bono esse: "serve as evidence", "serve as something good" is a good translation, so I understand why you call that a dative of service. It is related to the dative of purpose but slightly different in meaning. My dictionary also says:

argumento esse als bewijs dienen ["serve as evidence"]

I would say both infinitive constructions are accusativi cum infinitivis.

ut magno argumento esse debeat causam et principium philosophiae esse inscientiam

"...this should serve as strong evidence that ignorance is the cause and beginning of philosophy"

Praedicates expressing a meaning similar to saying, thinking, etc., are often followed by an a.c.i., as of course you well know, and compound expressions with esse can also work this way. E.g. certum est Iphigeniam morituram esse, "it is certain that / it is my decision that Iphigenia shall die" (I'm actually not entirely sure whether a future works here, but that is not relevant to the point).

The subject is indeed implicit, "this/it", referring to the preceding main clause: "the most learned men have so many different and conflicting opinions", "that this should be strong evidence that...". It looks ugly in English because we translate both ut and the a.c.i. by "that".

Quod autem idem maestitiam meam reprehendit, idem iocum, magno argumento est me in utroque fuisse moderatum.

"But that the same man should find fault with my sadness, and also with my jocularity, serves as evidence that I was moderate in either."