In Reformed Dogmatics, Geerhardus Vos explains predestination in a most wonderful way. Here’s part of it (edited very slightly):
The doctrine of human inability after the fall is inseparably connected with predestination, so that one must maintain them both together or drop them both together. One of the two; it depends on God or it depends on man who will be saved. If one chooses the first, then one has accepted predestination. If one chooses the second, then this is possible only under two presuppositions:
1) Either one must be Pelagian and say man has not become lacking in ability through his fall; then he is able to decide for himself.
2) Or one must say man was at first lacking in ability, but God does something to man, in fact to all men without distinction, whereby they again become able to make a decision. But then A, who is saved, must have something good that B, who is not saved, does not have. And A must have this good of himself because he received precisely the same grace as B.
In any case, here the inability of man is denied, whether one propounds that denial in a Pelagian or a semi-Pelagian fashion.