Summa contra Gentiles1.83 Chapter LXXXIII. That God wills something other than himself by a necessity of supposition

We may conclude from the foregoing that, although God wills none of His effects of absolute necessity, He wills something necessarily by supposition.

For it has been proved that the divine will is unchangeable. Now that which is once in an unchangeable thing cannot afterwards not be therein: since we say that a thing is changed when its condition is different now to what it was before. Therefore, if God's will is unchangeable, supposing that He will something, it is necessary by supposition that He will it.

Again. Everything eternal is necessary. Now that God will some particular effect to exist is eternal: for His willing, like His being, is measured by eternity. Therefore it is necessary. Not however if we consider it absolutely: because God's will has not a necessary relation to this particular thing willed. Therefore it is necessary by supposition.

Further. Whatsoever God could do, He can do, for His power is not diminished, as neither is His essence. But He cannot now not will what He is supposed to have willed, since His will is unchangeable. Therefore He never could not will whatever He has willed. Therefore it is necessary by supposition that He willed, as also that He will, whatever He willed: neither however is necessary absolutely, but possible in the aforesaid manner.

Moreover. Whosoever wills a thing, necessarily wills those things which are necessarily requisite to that thing, unless there be a defect on his part, either through ignorance, or because he is led astray from the right choice of means to the end in view, by some passion. But these things cannot be said of God. Wherefore if God, in willing Himself, wills something other than Himself, it is necessary for Him to will all that is necessarily required for what is willed by Him: even so is it necessary for God to will that there be a rational soul, supposing that He wills a man to be.