Jacques Maritain Center
The movement of the heavenly bodies [i.e. of the crystal
spheres which carry sun, moon and stars] is uniform [semper
est eodem modo]. If then the effects of the heavenly bodies on
these sublunary bodies were produced of necessity, the phenomena
of sublunary bodies would be uniform. But they are not
uniform, but happen only for the most part. Therefore they
do not happen of necessity.' (Contra Gentiles, iii. 86, n. 3).
This characteristic bit of ancient Scholasticism is explained in
Of God and His Creatures, notes on pp. 184, 254.