Short answer. A gift to an unknown person is void, but a gift to a defined group is valid. Article 845 of the Civil Code voids a disposition in favor of an unknown person, unless some event or circumstance later makes his identity certain, while upholding a disposition in favor of a definite class or group.
What the law says
a disposition in favor of a definite class or group of persons shall be valid.
Civil Code, Article 845 — Dispositions to Unknown Persons. Read the full provision →
An unknown person: void, with one saving path
Article 845 begins with a firm rule: Every disposition in favor of an unknown person shall be void, unless by some event or circumstance his identity becomes certain. A will must be able to identify who takes. If the beneficiary is left so vague that no one can tell who was meant, the gift fails. The single escape is that some event or circumstance later makes the person's identity certain — at which point the disposition can take effect because the recipient is no longer truly unknown.
A defined class or group: valid
The article then draws the contrast: However, a disposition in favor of a definite class or group of persons shall be valid. You may leave property to a group — described by a shared characteristic that marks its members — even though you do not name each person. The key word is definite. The class must be defined clearly enough that its members can be ascertained. A gift to a recognisable, bounded group stands; a gift to a hopelessly indefinite one does not.
Drawing the line
The dividing line in both halves of the article is certainty of identity. An unnamed individual is fatal only if he cannot be identified at all; a group is fine as long as it is definite. What the article does not do is bless loose or open-ended language that leaves no way to know who benefits. Because whether a description is definite can be contested after you are gone, describe any class carefully and, where an individual is meant, give enough detail to fix who it is. Careful drafting is what keeps such a gift from being struck down.
How a class gift is worked out
A few practical consequences follow from the certainty test. With a class gift, someone still has to determine who actually falls within the described group, and as of the time the will fixes for that determination; a clear description makes that exercise straightforward, while a vague one invites dispute. If a supposed class turns out to have no ascertainable members at all, there is effectively no valid beneficiary and the gift cannot take effect. None of this displaces the reserved shares the law guarantees compulsory heirs, which stand whatever group you choose to benefit with the free portion of your estate.