In virtue of reason man is capable of knowing the other as other. Brute animals can only know as desirable, repulsive or ignorable. The brute knows only with respect to its own good. Man distinctively moves beyond such desire to the theory of reality as that which exists independent of our desire and cognition.
Other as known as other is then distinctive of the rational animal and particularly of his distinctive power. In virtue of this man can desire the good of the other as other.
The desire of the good of the other qua other is particular to man and represents a certain perfection. Brutes desire their own good but this move of rational love extends the good in such a way that the good becomes increasingly transcendent and thus greater and wider. The love of other as other is an eminent good for the lover as in embracing a transcendent good (meant here as in an initially limited sense of other men rather than necessarily the Supreme Transcent Good) that increases his own good the wider and greater the intensity of his love.
The rational animal has than an unbounded capacity or desire for seeking the good of the other and this is his distinctive good as rational.