Before us is Mishnah 9 in chapter 3 of Maseches Shevi'is. The Mishnah deals with the question of where a person may take stones for building purposes, and under what conditions he will not appear as one who is improving and preparing his field.
"Avnei kaseif ba'os mikol makom":
Shoulder stones are heavy stones that are carried on the shoulder, and the Mishnah rules that they may be brought from anywhere - even from a person's own field. The reason: since these stones are heavy and require a shoulder to carry them, it is clear to all who see that he is taking them for building purposes, and not in order to improve and prepare his field. We will return to the precise definition of these stones later in the Mishnah.
"V'hakablan meivi mikol makom":
A building contractor may bring stones from anywhere, even very small ones. The reason: since building is his profession, everyone knows that he is taking the stones for building needs and not for any other reason.
"V'eilu hein avnei kaseif":
How does the Mishnah define shoulder stones? On this the Tanna'im disagree:
Rabbi Meir: "Kol she'eino yachol linatel b'yad achas" - any stone that a person cannot carry with one hand alone and pick up easily. This is a simpler and more convenient definition.
Rabbi Yosei: "Avnei kaseif k'shman" - their name teaches about them, and from the plural form it emerges that there is no requirement of a single stone: "Kol shehein nitalos shtayim shalosh al hakaseif" - it is enough that he carries two or three of them on his shoulder, and even a greater number of stones is included in this.
Reconciling this with the halachah we learned earlier:
We learned earlier that one who takes stones from a fence must take stones each of which is a load for two people - a stone that only two men can carry. How does that halachah fit with the definitions before us?
That halachah was stated where one wishes to take even the small stones; but here we are dealing only with taking the shoulder stones themselves, and for that there is no need for a stone the size of a load for two people - one stone on the shoulder suffices according to Rabbi Meir, or several stones on the shoulder according to Rabbi Yosei.
In summary: In this Mishnah we learned that shoulder stones may be brought from anywhere, even from a person's own field, since their weight proves that they were taken for building and not for preparing the field; a contractor may bring from anywhere, even small stones, since his profession proves his intent; and regarding the definition of shoulder stones, Rabbi Meir holds that it is any stone that cannot be carried with one hand, while Rabbi Yosei holds that it is any stones of which two or three are carried on the shoulder. We also noted that the requirement of a load for two people was stated specifically where one comes to take the small stones as well.