D&C 68: 1-6:
1 MY servant, Orson Hyde, was called by his ordination to proclaim the everlasting gospel, by the Spirit of the living God, from people to people, and from land to land, in the congregations of the wicked, in their synagogues, reasoning with and expounding all scriptures unto them.
2 And, behold, and lo, this is an ensample unto all those who were ordained unto this priesthood, whose mission is appointed unto them to go forth --
3 And this is the ensample unto them, that they shall speak as they are moved upon by the Holy Ghost.
4 And whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the power of God unto salvation.
5 Behold, this is the promise of the Lord unto you, O ye my servants.
6 Wherefore, be of good cheer, and do not fear, for I the Lord am with you, and will stand by you; and ye shall bear record of me, even Jesus Christ, that I am the Son of the living God, that I was, that I am, and that I am to come.
The Holy Ghost is a member of the Godhead. The Holy Ghost is God. Whenever God speaks, it is scripture. Therefore, whenever God chooses to speak through mortal men or those "arm of flesh" dudes, then it is scripture.
Our job is to find the true prophets of God, and then learn to detect when they are being moved upon by the Holy Ghost to speak, and when they are not.
My current Bishop has the annoying habit of never giving me anything that I want, but he always seems to be able to give me exactly what I need, because he speaks to me while being moved upon by the Holy Ghost to speak, and a lot of what he says to me is scripture.
Following the natural man will not get you into heaven, but following the prophets of God will. The "natural man" is the arm of flesh talked about in the scriptures. In contrast, the "spiritual man" is a Prophet, because he speaks God's Word whenever he is moved upon by the Holy Ghost to do so. There's a difference there.