The Holy Ghost
The Still Small Voice - A story by Wilford Woodruff about listening to the spirit.
When a man has the manifestation from the Holy Ghost, it leaves an indelible impression on his soul, one that is not easily erased. It is Spirit speaking to spirit, and it comes with convincing force. A manifestation of an angel, or even the Son of God himself, would impress the eye and mind, and eventually become dimmed, but the impressions of the Holy Ghost sink deeper into the soul and are more difficult to erase. - Joseph Fielding Smith
Example of the Spirit's teaching - two short stories related by Harold B. Lee illustrating the teaching power of the Spirit.