The use of certain tools on Shabbat is discussed. The Mishnah, for example permitted several, for example:
A MAN MAY TAKE A HAMMER TO SPLIT NUTS. . .
Now, the Rabbis argue that what is being referred to is a specific tool:
Rab Judah said: [This means,] a nut hammer to split nuts therewith, but not a smith's [hammer]
Because, he argues, a smith’s hammer has a function which is forbidden on Shabbat – therefore it cannot be used even for something permitted. Others argue that it is the function, not the tool, which is forbidden, therefore a tool normally used for a forbidden function can be used for a permitted one.
The next Mishnah is far more general:
R. JOSE SAID: ALL UTENSILS (including tools) MAY BE HANDLED, EXCEPT A LARGE SAW AND THE PIN OF A PLOUGH
What’s going on? Couldn’t that have been stated before?
Our Rabbis taught: At first they [the Sages] ruled, Three utensils may be handled on the Sabbath. . .Then they permitted [other articles], and they permitted again [still more], and they permitted still further, until they ruled: All utensils may be handled on the Sabbath except a large saw and the pin of a plough.
There seems to be some evolution in the text. The Rabbis explain it this way:
R. Hanina said: This Mishnah [permitting only three specific utensils] was taught in the days of Nehemiah the son of Hacaliah, for it is written (Neh 13:15), In those days I saw in Judah some treading winepresses on the Sabbath, and bringing in sheaves.
In other words, in a time when people were lax about the laws, the rabbis enacted more restrictions. They were eventually loosened.
This counters those who think that the law is and always was. Jewish law is, in fact, responsive to its time. It is intended as the tool for the job at hand.