Responsa for Shabbat 162:5

רבא מתני לה משום השרת נימין וקשיא ליה דר' יוחנן אדר' יוחנן מי א"ר יוחנן אסור לקנח בחרס בשבת אלמא דבר שאין מתכוין אסור והאמר ר' יוחנן הלכה כסתם משנה ותנן נזיר חופף ומפספס אבל לא סורק אלא מחוורתא כדרב נתן בר אושעיא

Then on weekdays too [let it be forbidden]? Again if it is on account of witchcraft:<span class="x" onmousemove="('comment',' As below. ');">¹⁶</span> it may not [be done] even on weekdays too? Again, if it is on account of the tearing out of hair, — but surely that is unintentional? — Said R. Nathan b. Oshaia to them: [Since] a great man has stated this dictum, let us give a reason for it. [Thus:] it is unnecessary [to state] that it is forbidden on weekdays;<span class="x" onmousemove="('comment',' Since one can just as easily take a chip or a pebble, to which no suspicion of danger or witchcraft attaches. ');">¹⁷</span> but on the Sabbath, since it bears the rank of a utensil, [I might think that] it is permitted:<span class="x" onmousemove="('comment',' Being preferable to a chip or a pebble, which are not utensils, and in general it is permitted to handle a utensil sooner than that which is not a utensil. ');">¹⁸</span> therefore he informs us [otherwise].

Teshuvot Maharam