“A true friend is, as it were, another self.”
The phrase captures the intimacy of friendship: another person becomes close enough that concern for them resembles concern for oneself.
Source details
De Amicitia, chapter 21 / classical section 80, Andrew P. Peabody translation, Project Gutenberg #7491. Laelius says that unless self-love is transferred into friendship a true friend cannot be found, “for such a friend is, as it were, another self.” The familiar another-self formulation is presented in its philosophical context and not as a literal identity claim.