Paul's mind was produced by a three-pound lump of flesh in his head. Seeing
as how no good thing dwelled in Paul's flesh, then according to Rom 7:18,
the mind that Paul's three-pound lump of flesh produced was not a good mind.
Soul and body are closely related. But distinct. 1 Thes 5:23. The 'mind of the flesh is death,
the mind of the spirit is life and peace.' We're tripartite, 3 basic parts. Sin dwells in my flesh,
however mysterious that is, and influences my soul. As an unbeliever, our spirit is dormant,
empty, dead. You're right that there is none good but God.
But Paul used the word 'good' in Romans 7. The anti-source, the tree of knowledge of good and
evil, knows good and evil, but is not God. So there's a difference between knowing and being.
Saul (and I) know what is good, even know what the law what is of God. The problem is in being
it. We can't be it. So we have to mind the spirit in Romans 8. Because it not only can be good
('the righteous requirement of the law is fulfilled in those who walk according to the spirit'),
it is 'God.' Mingled with God. Romans 8:16, 10; 1 Cor 6:17