The proximate differentia expresses the essential feature by which one species is distinguished from other species sharing its proximate genus. Traditional logic uses nāṭiq (“rational”) for the human species.
In this technical usage, nāṭiq does not merely mean speaking aloud; it refers to the intellectual capacity characteristic of humans. Someone who cannot speak is still human.
“Animal” identifies the wider class; “rational” gives the specific essential distinction. Their combination forms the complete essential definition.