I saw a Latin inscription in a church in Rome years ago, and there was an interesting feature. It mentioned a pope and his filius. We were a couple of Latinists and we agreed that so it said, but we didn't know why. There seemed to be two options:

  1. The pope had a biological son.
  2. The other person might be the pope's protégé of some kind.

For most people the first option is more likely, but we thought that popes are less likely to have a son than most other people. This likeliness may have varied over time, but this is not actually important to my question. I would like to know if the second option would really be a possible reading of the word filius.

Does filius always refer to a son or some other biological descendant, or can it also refer to a protégé of some kind? Is such non-biological use, if it exists, restricted to adoptive sons? For example, can a teacher refer to his student as a filius if there is no family relation between them whatsoever? I am only interested in a filius of a person, not expressions like fortunae filius.

Lewis & Short seems to say that a person's filius is a biological descendant, most typically a son. However, the inscription was certainly not from classical antiquity, so the definition of L&S might not have been valid when it was inscribed.

  • Can you remember the whole inscription?
    – Ben Kovitz
    May 5 '17 at 8:14
  • @BenKovitz No, alas. I clearly remember wondering about the problem, but I don't remember the inscription, any dates, or the church. It was six years ago. Because of this leaky memory I wanted to ask the question so that it doesn't matter.
    – Joonas Ilmavirta
    May 5 '17 at 8:46
  • 2
    FWIW, Caesar reportedly called Brutus fili mi regardless of being his adopted son. In the Vulgate (and also in Ecclesiastical Latin) there are also many examples of filii by adoption: example.
    – Rafael
    May 5 '17 at 12:11
  • Also not necessarily in a legal sense: Timotheo germano filio in fide (1 Tim 1,2.)
    – Rafael
    May 5 '17 at 12:17
  • 3
    All language is so steeped in metaphor I'd be very surprised if there's a language in which son cannot be figurative.
    – cmw
    May 5 '17 at 14:28

Short answer: no. At least since Post-Classical Latin, and quite possibly from earlier.

  • One may or may not believe the quote attributed to Julius Caesar when he calls Brutus fili mi despite the fact their relationship was aquired by adoption. Nevertheless, it is a sign that at some point later, it was considered valid and no one saw it as a mistake.
  • The Vulgate uses filius both to mean an adopted child and an arguably wider kind of spiritual relationship where someone is brought to the life of faith by someone else.
    • St. John says we have become children of God: e.g. dedit eis potestatem filios Dei fieri (Ioh 1,12). This has been always interpreted as a form of adoption: cf. e.g. Basil of Caesarea (IV century), and Augustine of Hippo.
    • John and Paul (among others) call disciples filioli and filii: e.g. I Ioh 2,1 and I Tim 1,2.
  • In Christianity, Deus Filius (Jesus) is one of the three persons of the Holy Trinity, the One God, and that of course is not a biological filiation to the person of God the Father.

Regardless of that, it is worth mentioning that there have been Popes with biological children, both legitimate and illegitimate. Although celibacy has been the norm for bishops in the Catholic Church since at least very early, there have been widowed men that become Popes, as well as married ones exceptionally allowed to.

Possibly the best example of a Pope with a legitimate child is Saint Hormisdas (VI century), who was married and become widowed before he took the Holy Orders: one of his children also became Pope thirteen years after his death and is today known as Saint Silverius.


I agree with Rafael's answer. Here are some specific classical examples to support this:

  1. Filius can refer to an adopted son

    Filiorum neque naturalem Drusum neque adoptivum Germanicum patria caritate dilexit, alterius vitiis infensus. (Suetonius, De Vita Caesarum Tiberius 52)

    And an example from Gaius the jurist (2nd c. AD):

    Adoptiui filii, quamdiu manent in adoptione, na- 136.1 turalium loco sunt. (Gaius, Institutiones 2.136)

  2. Filius can be used figuratively

    extra communia censes
    ponendum, quia tu gallinae filius albae,
    nos viles pulli, nati infelicibus ovis? (Juvenal, Satire XIII, 140-142)


    per totum hoc tempus subiectior in diem et horam
    invidiae noster. ludos spectaverat, una
    luserat in campo: 'Fortunae filius' omnes. (Horace, Satires 2.5)

Although I spent a lot of time looking, I still have not found a classical example of filius being used as a term of affection. I will add if I find anything.

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