NUM176 "You must not explore after your heart and your eyes" (this verse). We have herein been admonished not to contemplate [the previous paragraph involved fantasizing, which can bring one to a state of impurity. The current prohibition has greater ramifications; it forbids thought directed toward actually committing a sin, which can lead to one's intentionally performing the sin.] committing a transgression, any form of inequity or sin, as the pasuk says (Mishlei 24:8): "One who thinks of doing evil," one must also not reflect upon the words of heretics, lest he stumble and be drawn towards them. When one impresses on his heart that Hashem explores the heart and examines man's innermost thoughts, will he dare defile his heart by filling it with lawlessness? [Tehillim 41:9]. Shlomo, a"h, also said (ibid., 9) "the thoughts of a fool are sin"; and he further said (ibid., 6:16-18), "These six Hashem hates, and for the seventh He holds a deep abhorrence… a heart that contemplative sinful thoughts." [this is actually the fourth listed in the verse; the seventh is "one who sows discord among brothers."]