Second Nephi, chapter 2 contains 4 major philosophical and theological truths that are worth mentioning. They are (1) the symbiosis of good and evil, (2) the irrational implications of moral relativism, (3) the essential nature of moral agency, and (4) the symbiosis of joy and misery.
1. The symbiosis of good and evil. “For it must needs be that there is an opposition in all things. If it were not so . . . righteousness could not be brought to pass. . . . Wherefore, all things must needs be compound in one.” (verse 11)
To a certain extent righteousness is defined by its opposite, evil, and evil is defined by its opposite, righteousness. Righteousness is what it is because it is contrasted with evil, and evil is what it is because it is contrasted with righteousness. When Lehi wrote that all things must be compounded into one, he was saying that if one ceased existing, the other would cease existing as well. In modern day language we might say that righteousness and evil are two sides of the same coin. One side of the coin relies on the other side for its existence. Consider that if you removed the “heads” from a coin it would no longer be a coin, and thus the “tails” would cease to exist as well. There are no "heads" without "tails", and there is no righteousness without evil.
2. The irrational implications of moral relativism. “And if ye shall say there is no law, ye shall also say there is no sin. If ye shall say there is no sin, ye shall also say there is no righteousness. . . . And if there be no righteousness . . . . [then] there is no God. And if there is no God . . . there could have been no creation . . . [and] all things must have vanished away.” (verse 13)
Moral relativism claims that there is no divine law governing what is good and bad behavior, and thus any behavior that feels good and does not hurt others is appropriate. Relativism also asserts that, because there is no law, there is no sin or evil. However, as Lehi pointed out, if there is no sin then there could be no righteousness because they are opposite sides of the same coin, and without righteousness there could be no God because He is perfect and glorified. Finally, if there were no God there could be no creation and everything would vanish into nothingness. But the universe is not in a state of nothingness because we know we are here. The reality of our existence puts moral relativism on a shaky foundation. When carried to its logical conclusion, it is an intellectually bankrupt enterprise.
3. The essential nature of moral agency. “Wherefore, man could not act for himself save it should be that he was enticed by the one or the other.” (verse 16)
In this verse Lehi is referring to the power to choose between good and evil (moral agency). There are two essential influences that must exist for mankind to exercise its moral agency and thus qualify for a kingdom of glory – they are the influences of good and evil. We cannot qualify for exaltation without being enticed by evil influences (and eschewing them), and we cannot qualify for exaltation without being enticed by righteous influences (and choosing to follow them). Evil enticements come from Satan and his followers, and righteous enticements come from the Spirit of the Lord. Both are necessary for agency to exist.
4. The symbiosis of joy and misery. “[W]herefore they would have remained in a state of innocence, having no joy, for they knew no misery; doing no good, for they knew no sin.” (verse 23)
This verse refers to the essentiality of the Fall of Adam. Adam and Eve could only feel true joy after falling from the presence of God and experiencing the misery that comes with telestial, mortal existence. And so it goes with us. Living through mortality with all its misery and suffering is essential if we are to truly understanding the joy that comes with the Plan of Redemption. The Plan of Redemption is sometimes called the Plan of Happiness, and for good reason. Those who experience redemption after enduring the trials of mortality will experience immense happiness. This concept can be extended to other areas of life, such as that we appreciate the joys of health through sickness and we appreciate the joys of success through failure. Of course, we do not need to purposely create misery, sickness, and failure to experience their opposites, these things just seem to find us in mortal existence.
In many respects, these profound ideas are superior to contributions from more recognizable philosophers who came centuries later. I think this makes Lehi one of the most under recognized philosophers in the history of the world.