Taharos, Chapter 6, Mishnah 10. This is the closing mishnah of the sixth chapter, the chapter that has been sorting out the domains, the reshuyos, and how each one behaves when a case of doubt arises. Along the way the chapter has taken us on something of a tour of the architecture of the ancient world, the Greek-influenced structures that filled the cities of that period, and it now finishes that tour with one final building.
The mishnah's subject is the istvanis: "ha-istvanis." An istvanis is a colonnade. If the word does not immediately conjure a picture, it is worth looking up images of one, because some of them are stunning: a long, narrow walkway with rows of columns running down each side, holding up a roof above.
Of such a place the mishnah rules: "reshus hayachid leShabbos u'reshus harabbim letumah." For the laws of Shabbos it is treated as a private domain, meaning practically a karmelis, and for the laws of tumah it counts as a public domain. Why is it public with regard to tumah? Because this is precisely the sort of place where the marketplace was set up, or where the crowds gathered as a public forum. People were streaming through it constantly, and it is the constant presence of the public that gives an area the status of reshus harabbim for the purposes of impurity.
The mishnah then adds a second case: "chatzer sheharabbim nichnasin bazu veyotz'in bazu." A courtyard with two entrances, where the public walks in through one side and out through the other. Here too the ruling is the same. It is a reshus hayachid for Shabbos, which again in practice means a karmelis rather than a genuine private domain, and it is a reshus harabbim for tumah, since a steady flow of people is passing through it in one side and out the other.
The principle behind both halves of the mishnah is one we have been building all chapter long. Shabbos and tumah measure a place by different yardsticks. Shabbos asks about walls and partitions and the physical shape of the space. Tumah asks a simpler question: who is here? Wherever the public actually walks, the area is public for tumah, whatever its architecture may say.
With this, the sixth chapter of Mishnah Taharos is complete, tam venishlam.