Note 21 at Gal. 3:15: Paul is now using a human illustration. If a man's covenant, contract, or agreement was drawn up and ratified, i.e. whatever was necessary to give it legal force had been done, then no man could break the agreement or add conditions to it. Likewise, God's covenant with Abraham was in force 430 years before the law of Moses (v. 17) and was superior to Moses' law. Indeed, Paul establishes in the following verses, that the law of Moses was only a temporary addition which would cease to have any effect once the promised seed (v. 16) came.