The spirited horse, which will try to win the race of its own accord, will run even faster if encouraged. - Ovid

Si nec blanda satis, nec erit tibi comis amanti,
Perfer et obdura: postmodo mitis erit.
Flectitur obsequio curvatus ab arbore ramus:
Frangis, si vires experiere tuas.
Obsequio tranantur aquae: nec vincere possis
Flumina, si contra, quam rapit unda, nates.

Venus of Eryx, from her mountain throne,
Saw Hades and clasped her swift-winged son, and said:
'Cupid, my child, my warrior, my power,
Take those sure shafts with which you conquer all,
And shoot your speedy arrows to the heart
Of the great god to whom the last lot fell
When the three realms were drawn. Your mastery
Subdues the gods of heaven and even Jove,
Subdues the ocean's deities and him,
Even him, who rules the ocean's deities.
Why should Hell lag behind? Why not there too
Extend your mother's empire and your own....?

Then Cupid, guided by his mother, opened
His quiver of all his thousand arrows
Selected one, the sharpest and the surest,
The arrow most obedient to the bow,
And bent the pliant horn against his knee
And shot the barbed shaft deep in Pluto's heart.

Causa latet, vis est notissima