What do the words "tunc tantum" mean together?

Question

Pope Francis tweeted on September 21st,

Sermonem confero cum aliquo sincerum tunc tantum agnosco illum esse donum Dei mihique aliquid pretiosum dicturum.

Here is my translation. (Credits to Keith Massey for his help.)

I engage in a sincere dialogue with someone only after I recognize that person is a gift of God and about to say something precious to me.

My confusion is with the phrase tunc tantum. I recognize tantum as an adverb meaning "only", but I'm unsure of how to translate tunc alongside of it. I am accustomed to tunc meaning "then", but in this case could it possibly mean "when"?

I checked the Lewis and Short dictionary, and couldn't find this sense of tunc. But it seems to be the most appropriate translation. Is going from "then" to "when" not such a big stretch after all — am I merely nitpicking? Or are there dictionaries that document this sense of tunc after all?

Answer 1

This usage strikes me as strange, so I will make the case for that. I would be happy if another answer corrects me!

There are 12 matches in the Packhum corpus database for tunc tantum: there are no cases of it being a temporal/conditional conjunction. The vast majority are coordinated with temporal conjuctions (tunc...cum / ubi / dum), one is used alone to mean "only at this time," and a final one is used with quanto + comparative adjective to mean "this only occurs the more X something is."

The Lewis and Short entry does list some conditional meanings of tunc, but it is always used as an adverb. An example:

cuperem tecum communicare tam subitam mutationem mei: tunc amicitiae nostrae certiorem fiduciam habere coepissem. (Sen. Ep. 6, 2)

I would like to discuss with you such a sudden change in me: then I would begin to have stronger confidence in our friendship.

Conclusion: tunc can only be used as an adverb

The current usage is different though: it essentially functions as a restrictive particle like dummodo, dumtaxat, or quatenus.

A simplified sentence that illustrates this use would be:

Bonus sum tunc tantum fidelis sum.

I am good only when (?) I am faithful.

Another points that makes me question this usage is that I expect the subjunctive for this kind of qualification, whereas agnosco is indicative. The other restrictive conjunctions mentioned above take the subjunctive as well, which seems to confirm this gut feeling.

Again, I am happy to be proven wrong.

Answer 2

As it is virtually impossible to draw up a complete list of every adverbial phrase, the rarer ones are often omitted from dictionaries, this perhaps being justified as hapax legomenon. It's sometimes necessary to use a bit of imagination, as many scholiasts or commentators seem to have done quite often in such circumstances.

I think here that tunc tantum simply means "only then", but with a little licence you might express it as, say, "at which point" - or construe into something else which might fit the case. Provided that it agrees with its context, you could even translate as "It is only when I'm engaged in serious dialogue with anyone that I can see that he is a gift, etc."