Lies remove you from rationality
While you are dishonest, you can’t also be rational. Lies rationalize what would otherwise be obviously irrational behavior. You can’t turn away from your ability to reason without losing some of your ability to reason.1
Lying makes you irrational and stupid, and can do so permanently. Any lie will curtail your access to God’s spirit, which is what powers your understanding.2 Even “little” lies corrupt you.
8 And there shall also be many which shall say: Eat, drink, and be merry; nevertheless, fear God—he will justify in committing a little sin; yea, lie a little, take the advantage of one because of his words, dig a pit for thy neighbor; there is no harm in this; and do all these things, for tomorrow we die; and if it so be that we are guilty, God will beat us with a few stripes, and at last we shall be saved in the kingdom of God.
9 Yea, and there shall be many which shall teach after this manner, false and vain and foolish doctrines, and shall be puffed up in their hearts, and shall seek deep to hide their counsels from the Lord; and their works shall be in the dark.
10 And the blood of the saints shall cry from the ground against them.
11 Yea, they have all gone out of the way; they have become corrupted. (2 Nephi 28)
The more you lie, the dumber you will become.
Korihor provided a good example of how lying dulls one’s rational ability. He received a mission to preach from a supernatural visitor who first said people had gone astray from God but also said that there was no God (see Alma 30:53). The message he was given to share was that it was foolish to believe in Christ because “no man can know of anything which is to come,” (Alma 30:13) but he also taught that “[God] never was nor ever will be.” (Alma 30:28) This message consisted of elements that were obviously mutually exclusive. If there was no God, how could anyone go astray from him, and why would it matter if they did? If no one could know the future, how could anyone proclaim that Christ would never come? Neither Korihor nor his followers caught the contradictions, because none of them were seeking what was true . Instead, they were searching for beliefs that made them feel justified in doing what made them feel good in the moment.
Those whose highest priority is how they feel right now are particularly susceptible to proliferating lies about what they are doing, why they are doing it, what alternative choices exist, and the cost and benefit of each. For example, Korihor liked the idea of being important, and his followers liked the idea of doing whatever they pleased without feeling any guilt. When the devil told him that he had a special mission to “reclaim [the] people, for they have gone astray,” he happily taught the words he had been given: that “whatsoever a man did was no crime” (Alma 30:17) and “when a man was dead, that was the end thereof.” (Alma 30:18) He continued to teach the words given to him because they made people follow him and made him feel good, not because he had sufficient reasons to believe they were true:
...I taught them because they were pleasing unto the carnal mind; and I taught them, even until I had much success, insomuch that I verily believed that they were true; and for this cause I withstood the truth, even until I have brought this great curse upon me. (Alma 30:53)
It’s only possible to exchange rational arguments with honest people. Our rational and emotional facilities are typically contrary to one another. A liar will not easily admit the real reasons they do things. Those who try to help them either waste their time arguing against false reasons made up on the spot or repeated denials of their actual motive. When you are in an emotional state, you can’t be helped rationally. When you prefer emotions to reason, you remove yourself from the influence of others who might use persuasion to assist you in more correctly seeing the available options and cost and benefit of each.
Sustained lies will damage your rational capability to such a degree that they can and do cause severe mental degradation. In many cases, mental degradation that is often regarded as the effect of old age is actually just the long-term effects of willful dishonesty, as shown by the magnification of delusions established long before old age. As modern people age, they increasingly cling to increasingly irrational surrogates for the enduring meaning they fail to encounter in their lives until their personalities are dominated by delusions that are so obvious and intense that they are difficult for others to deal with. They lose access to the ability to learn and grow beyond the asymptote of their willful lies, lose the ability to see and feel anything else, and everything about them decays to fit into that prison cell of their own make. These symptoms can be averted in the lives of people who break the chains of dishonesty while they still can.