My father, of blessed memory, was a very good parent. (I told him that once when I was a teenager. He was really touched. I was surprised he didn't just know that. Now that I am a parent, I know why.) One thing that made him a good parent was that he never made empty threats. He never, ever, even once threatened to kill any of us, no matter how egregious our sin. He would however, in a very calm and measured tone (making it all the more terrifying) threaten to break our arm. I used to think to myself, "Well... he does have medical insurance, and he looks pretty serious." So I would desist.
The Torah gives us two ways to create our own restrictions on ourselves. One can create an new Torah prohibition by making either a נֶדֶר/vow or a שְׁבוּעָה/oath. They are very similar, but there are some technical differences. The one outstanding difference is that it is a Torah prohibition to make and empty/meaningless שְׁבוּעָה, whereas taking an empty/meaningless נֶדֶר is not a transgression, but it flies in the face of the Torah directive that you should stand behind and honor all of your words.
Oaths and vows are discouraged because... I mean, didn't the Torah already give you enough prohibitions and restrictions? Why would you add more? There is one good reason: you want to be better, but you need a little help. For example, when I wanted to lose weight, it was very difficult sometimes to not grab some ice cream or some of that yummy looking cheesecake. However, if I had eaten meat, I have a clear six hours of absolutely no desire for those dairy desserts. Imagine I wanted to stop smoking or stop looking at inappropriate images on the internet. I could vow to give $10 to the shul I don't go to for every lapse. That "little" Torah obligation might be enough to help me push through.
There are a couple of rules about how to make an effective נֶדֶר/vow or שְׁבוּעָה/oath. There are also ways to get yourself out of a נֶדֶר/vow or שְׁבוּעָה/oath taken in haste. Interestingly there are a few situations, enumerated and analyzed in the third chapter of Masechta נדרים, that just don't count. You can say the words, but nothing really happens. The following are some examples.
You are shopping for some dress slacks at a fancy store known to have inflated prices. You look at a nice pair, ignore the price tag, and tell the salesman, "You see these pants? I vow to never eat ice cream again if I pay more than 50$ for them!" The salesman takes them from you and says, "Feel that material! I vow to throw them into the incinerator rather than sell them for less 100$!" You say, "How about 75$?"; the salesman says, "Sold; cash or credit card?" You can go straight to Dairy Star on the way home, wearing your new pants that the salesman is not going to incinerate. Why? Because you both had 75$ in mind when you started. Your words didn't match your thoughts, but you both had the same intention. (The gemara explores cases where you didn't have the same intentions. Check it out.)
You come back from downtown Chicago after the Thanksgiving day parade and say, "Wow! I vow never to ride my bike again if I didn't see a throng like those who left Mitzrayim!" Don't worry, you can still ride your bike. You can't possibly see that number of people from one vantage point in downtown Chicago.
You come back from Lincoln Park zoo (no entrance fees and you can actually bike there from here) and say, "You won't believe this, but I vow to eat four pounds of raw okra every day for the rest of my life if I didn't see a snake like the beam of an olive press!" (Author's note: Ewww! I hate okra!) Don't worry, you don't have to eat a single bite of okra (Author's note: Huge sigh of relief.). There is no such thing as a snake shaped like the beam of an olive press (triangular in body cross section and a ridged back). Don't blame, that's how the Ran explains it.)
The Ran says the mishna enumerates both those last two because they don't take effect for similar, but different reasons. Neither could have happened; that's the similarity. The difference is that the first is just physically impossible, the second is because that thing is completely mythical.
Note to the wise: Before you attempt this, you really ought to learn the gemaras and Shulchan Aruch on oaths and vows. In the meantime, just keep saying בלי נדר.
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