Reference for Menachot 63:10

אמאי הכא סתומות והכא פתוחות דלמא להשלים

Similarly, if scrolls of the Law or tefillin had worn out, one may not make out of them a mezuzah, for one may not bring down what is of a higher sanctity to a lower sanctity.<span class="x" onmousemove="('comment',' The mezuzah is deemed to be of a lesser sanctity since it contains only two Scriptural portions, whereas the tefillin contain four. gna');">⁹</span> Now it follows that if it were permitted to bring down to a lower sanctity one would be allowed to make [a mezuzah out of tefillin or a scroll of the Law]; but how is this possible?

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