In their discussions about actions that are prohibited on Shabbat, the rabbis of the Talmud establish a new category: things that are prohibited merely because they appear to an observer to be forbidden. We saw an example of this on Shabbat 54a:
One may not tie camels one to another and pull the lead camel, causing the others to follow, like in a caravan. However, one may place the ropes tied to each camel in one’s hand and pull them.
In the Gemara, Rav Ashi explains why: when marched in a line, it appears that the camel driver is leading the camels to market, which would be forbidden on Shabbat. So as not to give any misimpression, one must use the alternate method of holding each camel’s individual lead in hand.
A second example also appears on Shabbat 54, where the mishnah teaches that it is prohibited for a donkey to go out for a walk wearing a bell on Shabbat, even if the bell is plugged to prevent it from ringing. The reason? Same as before: it would appear the animal is headed to market.
This principle, known as marit ayin, appears on today’s daf, in a slightly different way. The mishnah teaches that while a woman may not go into the public domain on Shabbat with a woolen cap or wig, she may go into a courtyard wearing these items. (A courtyard, in this case, is an enclosed space that is designated as a karmelit, neither privately owned nor public property.)
The Gemara quotes Rav, who interprets the mishnah as follows:
All ornaments and garments with which the sages prohibited going out to the public domain on Shabbat, it is also prohibited to go out with them into the courtyard, with the exception of a woolen cap and a wig.
Though the woolen cap and wig are exceptions (the rabbis allowed women these few adornments in the courtyard so they could look beautiful in the eyes of their husbands), Rav reports that the sages have established a general rule that items which may not be worn in a public domain are similarly disallowed in the courtyard. But if the courtyard is not truly a public domain, then why?
After some further discussion, Rav Yehuda quotes another teaching of Rav which explains the logic here:
Wherever the sages prohibited an action due to marit ayin (the appearance of a violation), even in the innermost chambers, where no one will see it, it is prohibited.
So various items that should not be worn in the public domain for fear of appearing to violate rabbinic law are also not to be worn in the courtyard.
One could argue that things that are prohibited for the sake of appearance should only be prohibited in public, but permitted in private where there is no one to view the action and make an erroneous assumption about it. Rav, however, reaches the opposite conclusion. If one is prohibited from an action in public because it looks wrong, one should not be allowed to perform that action in private.
Perhaps Rav is concerned that if one gets used to performing certain actions privately, one is more likely to perform them publicly. He has some insight into what behavioral science has also taught us (namely, that we are creatures of habit) and wants to prevent us from developing habits in private that are forbidden in public.
The principle of marit ayin is something the rabbis took seriously. What things look like matters, and we should take care not to lead others to draw erroneous conclusions based on casual observation. Rav, to protect us from potential wrongdoing, extends the principle to moments when no one is there to see.