We now begin the fourth chapter of Maseches Avodah Zarah, with the first Mishnah. The subject of this Mishnah is a particular form of idolatry known in the language of the Mishnah as 'Markulis'. Markulis is the Hebrew form of the Latin name Mercurius - the planet Mercury, and in its Greek version, Hermes. Historically, this deity was worshipped as the patron of commerce, good fortune and thieves.
How the Markulis was worshipped:
In the era of Tanach, its worshippers would set up a small mound of stones, especially at crossroads and junctions. The most basic form of the structure is three stones: two resting on the ground and one on top of them - the upper one serving as the head, and the two lower ones as supporting arms, and in this way the Markulis was represented.
Whoever arrived at the crossroads would ask that idol for good fortune, and the way he cast his lot with it and sought its favor was by adding a stone or throwing a stone onto the pile. That is how these piles of stones became Markulis idols.
The question of the Mishnah:
In this Mishnah, Chazal deal with the status of stones found near a Markulis pile. A pile dedicated to Markulis has the status of an asheirah - it is itself idolatry and is certainly forbidden to derive any benefit from it. The question is what the status is of stones found nearby: should we be concerned that this is a new Markulis that has been built? Or perhaps they fell off the old one? And what is their status regarding the prohibition of deriving benefit?
The view of Rabbi Yishmael:
"Shalosh avanim zu b'tzad zu" - three stones side by side are forbidden. If one finds three stones lying side by side at a distance of four amos or more from the Markulis, but still within sight of it, we must be concerned that a small new Markulis has been set up here. This is the beginning of a new structure, and therefore these stones are already considered stones of idolatry.
"U'shtayim" - but two are permitted. The basic form of the Markulis is two stones at the base with one resting on top of them; and if only two stones are found, there is no miniature Markulis here, and the stones are permitted - even though they are within sight of the Markulis and in its vicinity.
The view of the Chachamim:
"Es she'nir'os imo" - those that appear to belong with it are forbidden. Stones that look like part of the Markulis, meaning those found within four amos of it, may well have been placed or thrown there in order to become part of the pile, and therefore it is forbidden to derive benefit from them. And it makes no difference whether one stone was found, or two, or many: anything within four amos of the main structure is suspected of being part of the Markulis or of having fallen from it.
"Es she'ein nir'os imo" - those that do not appear to belong with it are permitted. Any stone more than four amos away from the main Markulis pile is permitted, even if three stones were found side by side, since we are not concerned that a small new Markulis was set up alongside the main one.
In summary: in this Mishnah we learned about the Markulis - an idolatry made of piles of stones at crossroads, to which worshippers would add a stone in order to request good fortune - and about the status of the stones near it. According to Rabbi Yishmael, three stones side by side are forbidden out of concern for a new Markulis, while two are permitted; according to the Chachamim, everything depends on proximity: those that appear to belong with the Markulis are forbidden, and those that do not appear to belong with it are permitted. The halachah follows the Chachamim: only stones within four amos of the main Markulis pile are forbidden, at the very least out of doubt, as being part of the idolatry.