Paul was a person like you and me, but his writings such as Romans were inspired, inerrant words of God. So Paul is speaking the very mind of God here.
Oh yeah? I, freeeee, am hereby inspired by God. And I declare up to be down and black to be white. This is the mind of God himself. How do you know? Well, I told you so. Stay tuned for more pronouncements from God himself, spoken through my mouth of course.
See how it works? (BTW, before you get all upset about blasphamy, I'm just kidding to make a point)