DEUT1237 [This verse]. Our Sages, z"l, have commented (Kesubos 46a), "A person should not think [sinful] thoughts by day, thereby bringing him to a state of impurity at night," and this, even if he has no intent of acting upon them. This is derived from its context, for following this the pasuk states (Devarim 23:11), "If there is a man among you who is not pure because of a seminal discharge at night."
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