If someone for some reason can't eat a kezayais of matza, for example, should he at least have a mashe'hu to show his connection to mitzvos even though he is not mekayeim any Mitzvah at all – maybe a person who can't do a whole Mitzvah should do half a Mitzvah?
In the Birkei Yosef in Siman Taf Pei Beis – Birkei Yosef is the Chida's Sefer on Shulchan Aruch – he talks about someone who can only eat or only has a chatzi Shiur of Matza. He brings from the Shvus Yaakov in Teshuva Cheilek Beis, Siman Yud Ches who says that if you only have less than a Kezayis of Matza, there is no Mitzvah to eat it. Rav Moshe holds like the Shvus Yaakov, that there is no Mitzva B'etzem at all to eat less that a kezayis.
On this, the Birkei Yosef says that there is a K'tzas Mitzvah. Just as we find that you are not allowed to eat Nevailah that Chatzi Shiur Assur Min Hatorah; if you eat half a Shiur of Nevailah it is an Aveira as well. So just as Chatzi Shiur is an Issur D'oraissa, by a Mitzvah too there is an Inyan of a Chatzi Shiur being a Mitzvah.
The Kli Chemdah in Parshas Pekudai Os Daled discusses this. Anybody who remembers the Lomdus of the Sugya of Chatzi Shiur Assur Min Hatorah will realize that it has to do with that Lomdus. Why is a Chatzi Shiur Assur? The Gemara says Chazi L'itz'tarufai. That means that if you only eat half a Kezayis of Nevaila you are also Over an Aveira, since Chazi L'itz'tarufai, it can become an entire Zayis.
There are those who learn that Chazi L'itz'tarufai is a Sibah, and there are those who learn that it is a Siman. Meaning, some say since chatzi shiur can turn into a whole Kezayis, if you eat a whole Kezayis of Nevaila, then every part of that Kezayis is part of the Issur D'oraissa including the first part, so it must be that the first part is an Aveira.
Others say that it is just a Siman. The Torah says don't eat Nevaila, it means don't eat a bit of Nevaila, because Chazi L'itz'tarufai.
Lechora, the Nafka Mina between these approaches would be by a Mitzvah. If you say the first way, that Chazi L'itz'tarufai is the Sibah, the reason, so here also if a person eats an entire Kezayis, so every bit of the Kezayis is a Mitzvah, so Mimeila it is true that every part is a Mitzvah, and Chatzi Zayis should be a Chiyuv as well.
On the other hand, if it just a Siman L'davar, that the Torah doesn't want a person eating Nevaila, so that is by an Issur, but by a Kiyum Mitzvah there is no such Siman of Chazi L'itz'tarufai, and Mimeila, the entire Cheshbon, the entire calculation would actually not be accurate and it would somehow fall apart, so there is no inyan to eat half a shiur of matzah, for example.
Rabbi Yisroel Reisman
