Wed Nedarim 87

The Gemara says that Toch Kdai Dibur one may retract from his words except by Avodah Zara and Kiddushin. The Ran asks: what's the difference?

He Brings R' Tam who says that really from the Torah there is no such concept of retracting Toch Kdai Dibur. However, the Rabanan gave this Shiur so that a student who's making a deal, if at the end of the deal he meets his Rebbi and needs to greet him, he should still be able to retract afterwords. Although this doesn't apply to most cases, but the Rabbis said to make this Halacha universal so that we shouldn't need to differentiate between different Halachos. However, they decided to keep Kiddushin and Avodah Zara on their Torah Din, that there is no retractions within Toch Kdai Dibur.

The Ran disagrees. If someone made a Neder and retracted Toch Kdai Dibur, it's a Neder form the Torah, so how can the rabbis allow you to actively transgress that Neder? (We have a rule that the rabbis can only allow one to transgress a Torah law passively)

So the Ran explains: that most laws people are not certain they mean it until after Toch Kdai Dibur. If they don't retract by then, we know that they must mean it and it takes effect. However, Kiddushin and Avodah Zara which are very serious matter, if they had not thought it through in the first place, they would never uttered it. So the very fact that he said it shows he means it completely as soon as it leaves his mouth.

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