Galatians 3:20 - Wesley's Explanatory Notes

Now the mediator is not a mediator of one — There must be two parties, or there can be no mediator between them; but God who made the free promise to Abraham is only one of the parties. The other, Abraham, was not present at the time of Moses. Therefore in the promise Moses had nothing to do. The law, wherein he was concerned, was a transaction of quite another nature.