NUM198 It is a negative commandment not to go straying after the thought of the heart and the sight of the eyes as Scripture says, that you shall not follow after your own heart in your own eyes [this verse]. This means that we should not entertain in our heart any thought that can cause any one of the main principles of our faith to be uprooted; for Satan will incite us to heresy, Heaven forbid. And "after the sight of the eyes" means immorality. So the Sages of blessed memory taught (Talmud Bavli, B'rachoth 12b): "after your own heart" denotes heresy; "and after your own eyes," harlotry. Included in this prohibition is the rule not to plan and pursue other delights and cravings of this world. It is very, very necessary to be aware of reprehensible thoughts: for that is the foundation on which everything depends. For a man must hallow and purify his thoughts with all his might; and if some pleasure comes to him, let him have the intention that through it he should have the strength to stand in the palace of the Divine King, the Sovereign of the world, blessed is He; and that his body should not grow weak in the service of the Creator. If a person transgressed this and brought to mind reprehensible thoughts on matters of faith (Heaven forbid) or a fantasy of immorality, or [some plan] to pursue after material desires--it is a great sin that drives him out of the world-to-come since it defiles all the 248 limbs and parts, and the 365 sinews, of both the physical and the spiritual self. Therefore let a person greatly be aware of this.