Impure thoughts deaden our spirits but why else must we watch our thoughts? The Savior enlightens us through His intercessory prayer: "I have given them thy word; and the world hath hated them, because they are not of the world, even as I am not of the world. I pray not that thou shouldest take them out of the world, but that thou shouldest keep them from the evil. They are not of the world, even as I am not of the world. Sanctify them through thy truth: thy word is truth…. Neither I pray for these alone, but for them also which shall believe on me though their word; That they all may be one…even as we are one" (John 17:14-22). Thoughts that are in conflict with God's cannot lead to unity with Him. Impure thoughts cement our separation from the Savior and the Father because they are not sanctifying. Isaiah taught: "Let the wicked forsake his way, and the unrighteous man his thoughts: and let him return unto the LORD, and he will have mercy upon him; and to our God, for he will abundantly pardon. For my thoughts are not your thoughts, neither are your ways my ways, saith the LORD" (Isa. 55:7-8; emphasis added).
This is not to say that our thoughts cannot be like the Lord's; His thoughts are just not like the natural man's. If we forsake and remove our unrighteous thoughts, we can approach the Lord, just as Moses removed his shoes to approach the burning bush (see Ex. 3:5). So we need to have purity of thought in order to be one with the Father. This is also what the Lord revealed in Doctrine and Covenants section 88: "Therefore, sanctify yourselves that your minds become single to God, and the days will come that you shall see him; for he will unveil his face unto you, and it shall be in his own time, and in his own way, and according to his own will. Remember the great and last promise which I have made unto you; cast away your idle thoughts and your excess of laughter far from you" (v.68-69). From these verses we learn that idle thoughts, which are not necessarily evil, but might fall under the category of superfluous, also need casting away. Impurity of mind is not only the commission of impure thoughts but also the omission of pure ones. Pure thoughts are those of the truth, which includes the word of God. Whether we covertly think impure thoughts, or merely let our minds idle away, we are not pure in thought.