 Prove all things, hold fast to that which is good. What is good here? Well, if we go back to where he talks about here, verse 15, see that none render evil for evil unto any man, but ever follow that which is good. He's talking, follow that which is good. Then again, he's saying here, prove all things, hold fast to that which is good. He's talking about doctrine. Doctrine is what he's referring to here.