1:
In order to understand the situation described in our mishnah I must preface a few words of explanatory introduction. In very general terms, in the age of the Tannaïm the usual format for urban domicile was in the form of what was called a "courtyard". This is a term which simply means that a complex of several houses was built - usually in a U-shape - around a courtyard. The houses usually were of two stories, the lower one being called "the house" [bayit] and the one above it being called "the upper" [aliyyah]. Above the "upper" was a flat roof which was regularly used for many domestic purposes. The courtyard itself, around which the houses were arranged, belonged to all the householders in common. It usually contained communal amenities, such as a water-channel and a small mill. The householders would keep their livestock in the courtyard.

2:
The open end of the courtyard gave onto the street [reshut ha-rabbim]. Since the householders were very concerned to maintain their privacy the courtyard was usually separated from the street by a wall. So great was the desire for privacy that a mishnah [Bava Batra 1:5] even grants the majority of the householders the right to coerce a recalcitrant householder to chip in to cover the costs of building and maintaining the wall. In many cases they went even further: a guard, sitting in a small hut, was placed at the door in the wall that gave entrance into the courtyard in order to vet strangers who asked for access to the courtyard. (All the householders had to contribute to the wage of this guard, even under coercion.)

3:
It will, perhaps, be easier to understand the topography I have just described by looking at the artist's impression of a typical "courtyard" to the right. The "courtyard" of the time of the sages served the same purpose as the modern apartment block.

4:
When we studied the second mishnah of Chapter One of this tractate [BK 003] we learned that

When we apply this ruling to our present mishnah we can appreciate why the householder is not liable. Let us create an imaginary scenario.

Mr Kaduri is a potter and he goes round town trying to sell his wares. When he arrives at the entrance to the courtyard in which David lives the guard at the gate was not there. Mr Kaduri takes the opportunity to go into the courtyard to sell his wares. Technically he is now a trespasser. While he is bargaining with Sara, another householder, David's ox, Goliath, who was resting in the courtyard, accidentally brushes against Mr Kaduri's pots and they all fall off his cart to the ground and are smashed to pieces. Even if Mr Kaduri sues David to recoup his losses his action will fail, because David is not liable for damage caused on his own property to the property of a trespasser. Furthermore, if poor Goliath is injured by some of the broken pottery David can successfully sue Mr Kaduri.

5:
However, if David has invited Mr Kaduri to bring his wares into the courtyard he would be liable for damage caused to Mr Kaduri's pots by Goliath. Mr Kaduri, in such circumstances, is not a trespasser.

6:
At a first reading we might very well think that the second half of our mishnah is merely a repetition of what was already ruled in the first part of the mishnah, and therefore rather superfluous. Such a thought would not do justice to the acumen of the sages: there must be some difference, however small, between the first scenario presented by our mishnah and the second one.

7:
In the second scenario Mr Yarkan is a greengrocer and he goes round town trying to sell his wares. When he arrives at the entrance to the courtyard in which Leah lives the guard at the gate was not there. Mr Yarkan takes the opportunity to go into the courtyard to sell his wares. Technically he is now a trespasser. While he is bargaining with one householder, Sara's cow, Athaliah, who was resting in the courtyard, accidentally brushes against Mr Yarkan's cart and all the vegetables and fruit fall to the ground and are ruined. Even if Mr Yarkan sues Sara to recoup his losses his action will fail, because Sara is not liable for damage caused on her own property to the property of a trespasser.

8:
So far there is no difference between the two scenarios from the legal point of view. Our scenario now continues:

If Athaliah slips or trips on the produce which covers the ground Sara can successfully sue Mr Yarkan.

However, the Gemara [BK 47b] now points out an exception:

What Rav means is that the owner of an animal is required to take care at all times that his beast will not cause damage while doing something that can reasonably be foreseen. For example, as in our case, Sara cannot be expected to foresee that Athaliah will slip on the vegetables (which possibly should not have been there in any case); but she can be expected to know that Athaliah will eat any produce that comes her way, so Sara should muzzle her animal when it is not grazing.

In his commentary on our mishnah Rambam explains this last point thus: