Thinking Before Talking:
The Ben Ish Chai writes that when Hashem created man, He made two places in the body that are “the dwelling places of sense and understanding.” These places are the mind and the heart. The proper way to use these two tools is to first use the mind to think about what one wants to say before he says it. He should first consider if these words are worthy of being uttered and if he will be able to fulfill his word. Only once one has determined that the words are appropriate and that he can fulfill what he says should he use his mouth to actually say them. And once one says his words with confidence that they are appropriate, he should not forget what he said.
Rather, he should keep them in his heart and remember that he must fulfill his word.
He explains the concept of nedarim by saying that the Torah is telling us that one must first stop and think with his mind before he makes a vow, and he should only make it if he determines that he can fulfill it – for if one would blurt out the vow without thinking, he very well may not be able to keep his word.
Accordingly, the pasuk is saying: “A man who makes a promise or a vow” should be very careful not to “break his word” (“yachel devaro”). The words “yachel devaro” can also be translated to mean “to start with his words” – meaning that one should not start off with words. Rather, he should start in his mind by thinking if he can do what is about to say. If one does this, he can be assured that “everything that comes out of his mouth he will do”, as he will only make the vow if he is confident that he can keep it.
