The point that Mormon is making deserves some clarification. There are times when there is a point to praying when there is not yet full intent. Brigham Young said of such occasions:
“If I do not feel like praying, and asking my Father in heaven to give me a morning blessing, and to preserve me and my family and the good upon the earth through the day, I should say, “Brigham, get down here, on your knees, bow your body down before the throne of Him who rules in the heavens, and stay there until you can feel to supplicate at that throne of grace erected for sinners.” (Brigham Young. Journal of Discourses, 26 vols. [London: Latter-day Saints’ Book Depot, 1854-1886], 16: 28.)
While this might appear to be a contradiction to Mormon, the context of Mormon’s words tell us that there is a difference in meaning that is important. Mormon is beginning with a text from the Saviors Sermon on the Mount, as recorded in 3 Nephi. The Savior used several examples to show that one’s intent should be toward God, and not for the accolades of men. Mormon’s elaboration follows that theme. Prayer is not received of God if it is given for men. If that is its purpose, it might achieve that purpose, but not the higher function of communication with God. For that, we need to have true intent.
Brigham Young’s statement is slightly different. He is not addressing those who pray to impress others, but rather those who might neglect prayer that they know they should offer. Brigham Young is telling us that at times we must work to have the intent, even if it is not immediately present.