Another version of this story, referred to in the Book of the Dead, says Seth casts "filth" into the eye of Horus, causing it to emit liquid. In any case, Horus is always spoken of in terms of the regained strength of his eye, and Seth in terms of the loss of his virility.
Seth kills Osiris by cutting him into pieces and scattering them all over Egypt, but Isis, with the aid of her sister Nephthys, gathers the pieces back together and revives him long enough for him to impregnate her.
The river brings forth new life (Horus) in the form of vegetation.
Thus life wins out over death in a never-ending struggle.
This is reflected in the gender listing on the Middle Kingdom pottery shards of males, eunuchs, and females.
The word for male includes a picture of a penis and a picture of a man kneeling.
Through this doubling, shuffling, and dividing, a single FM being, such as Atum, can eventually produce beings which are MM (Osiris), FF (Isis), MF (Nephthys), and FM (Seth).