We learn in the Mishna that performing half of the melacha of carrying is an issur derabanan but not min hatorah. Why is it not assur min hatorah, since we know that even a ‘chatzi shiur assur min hatorah’ (i.e. eating half a kezayis of prohibited food)?
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The acharonim deal with this question. Some say that we see from here that Shabbos is different and we DON’T say chatzi shiur assur min hatorah; the reason being, ‘mileches machasheves’ is needed and a small amount is not chashuv.
Many others disagree and say there is a fundamental difference between chatzi shiur i.e. a small amount and a ‘half of a n issur’. Meaning to say if someone was to carry outside less than the shiur (less than a ‘grogros’) he Would be over min hatorah; however, doing an melacha but not doing all the steps i.e. ‘akirah’ without ‘hanachah’ is not assur min hatorah.A loose comparison would be putting food in your mouth but not actually swallowing it.