Commentary for Menachot 84:15

ת"ר תכלת אין לה בדיקה ואין נקחית אלא מן המומחה תפילין יש להם בדיקה ואין ניקחין אלא מן המומחה ספרים ומזוזות יש להן בדיקה וניקחין מכל אדם

Our Rabbis taught: There is no manner of testing the blue thread;<span class="x" onmousemove="('comment',' To ascertain whether it has been dyed with genuine or imitation blue.');">²⁴</span> it should therefore be bought only from an expert.<span class="x" onmousemove="('comment',' One who knows that vegetable blue or any other imitation blue is unfit for the purpose.');">²⁵</span> The tefillin can be tested,<span class="x" onmousemove="('comment',' To ascertain whether they have been made according to prescribed law and whether the Scriptural portions therein have been correctly written.');">²⁶</span>

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