2,754. Treating "Insubstantial" Vows Seriously
Hilchos Nedarim 3:11
Counter to the previous halacha, if someone says, “May it be prohibited for my mouth to speak, for my hands to act, for my feet to walk and for my eyes to sleep,” his vow is effective. Accordingly, if one person tells another, “My mouth is like a sacrifice for speaking to you, my hands for acting with you and my feet for going with you,” these activities are rendered prohibited. Similarly, if one person tells another, “I will be obligated in an offering if I speak to person X” or “if I don't speak to person X,” he must bring an offering for violating his word. The same is true if one took a vow saying, “If I spoke to person X,” “if I didn’t speak to person X,” etc. This is because these aren’t vows in which a person imposes prohibitions upon himself as we have been discussing; rather, these are vows of sanctification. (See halacha 1:2.)
Hilchos Nedarim 3:12
As we have discussed (in halacha 3:10), if someone takes a vow prohibiting things that lack substance, the vow is ineffective. Nevertheless, we don’t tell him to act as if such things are permitted. Since he intended to consider these things prohibited to him, the vow is quasi-effective. While the object of his vow is not truly prohibited, we still provide him with the opportunity to cancel the vow so that he won’t become accustomed to treating vows lightly.