On the bottom of Megillah 18b, there is a discussion as to whether Torah scrolls and other seforim can be written by a sofer without a source document. The question arises in the context of a discussion about how R' Meir traveled to a location where there was no Megillah, so he wrote one by heart.
At the end of the discussion, the gemara draws a distinction between seforim (such as a Torah) and mezuzah and/or tefillin. The gemara explains that tefillin or mezuzah can be written without reference to source materials since everyone is familiar with the pesukim.
For many years I have been receiving via email, R' Adler of the Beechwood Kollel's Daf Notes (also available on dafnotes.com). R' Adler had a great teshuva from R' Elyashiv in today's daf notes which illustrates this point.
Reb Elyashiv, cited by Chishukei Chemed ruled on the following inquiry and he derived his ruling from our Gemora.
A person residing in Eretz Yisroel developed a heart issue and he was required to undergo a risky heart surgery. The local doctors said that they do not perform this type of surgery frequently, but they are ninety-five percent confident that the surgery will be a success. There is an expert surgeon outside of Eretz Yisroel who performs this surgery daily and he said that if he would perform the surgery, he would be successful ninety-five percent of the time.
Should this individual travel outside of Eretz Yisroel to have the surgery performed by the expert surgeon? Reb Elyashiv ruled that he should go because that doctor is more accustomed in performing this type of surgery.
The Gemora in Taanis 15a states regarding a communal fast: They would send a chazzan to lead the prayer who was an elder and fluent in the prayers. Rashi explains: One who is fluent in his prayers will not make a mistake.
Our Gemora states that one who is fluent in the words of the Torah will not make a mistake when he is writing the Torah without copying from a text.
This is why it would be preferable to travel to the doctor outside of Eretz Yisroel even though the percentage of success is the same. Someone who is more accustomed to performing this type of surgery will not make a mistake.
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