ChaTzi Shiur, the Half Measure, What Is It That The Torah Prohibits?
Just as no contract can extinguish your legislated rights granted in the Trade Practices Act, or otherwise seeks to extinguish any rights granted by the government to its citizens, so too the Torah’s Laws cannot be over-ridden by an individual’s vow or promise.
Consequently, an oath taken to eat during Yom Kippur is not valid and not binding. However, an oath taken to eat less than the legal amount or to eat more than the legal amount but extended over a time that exceeds the maximum permitted time, is a valid binding oath.
This appears to make sense until we realise that we Pasken in accordance with Rebbi Yochanan, that a Chatzi Shiur is in fact, Assur Min HaTorah. Rebbi Yochanan argues this from a Sevara, a Chatzi Shiur is Chazi LeItzTaRufey – a small amount can theoretically be added to make up a full Shiur.
So how can it be that one can make a binding oath to eat less than a full measure of non-Kosher food during Yom Kippur?
The Meiri explains that the notion of Chazi LeItzTaRufey is applicable only when the intention from the outset is to eat a full Shiur. Planning to eat less than a full Shiur is not prohibited by Torah law and that explains how it is possible to make a legal and binding oath to eat on Yom Kippur, less than a full Shiur or even a full Shiur but extended beyond the maximum permitted time.
This might be analysed thus: when is the transgression actually transgressed? Is it only once the conditions have been satisfied, or perhaps once the conditions have been satisfied the transgression is activated retroactively, from the moment one began to eat?