Musar for Niddah 121:14
כל דבר שהוא בחזקת טומאה לעולם הוא בטומאתו עד שיודע לך טומאה היכן היא
THAT WAS MIXED UP WITH TWO CLEAN HEAPS, WHERE, IF THEY EXAMINED ONE OF THEM AND FOUND IT TO BE CLEAN, IT IS CLEAN WHILE THE TWO OTHERS ARE UNCLEAN; IF THEY EXAMINED TWO OF THE HEAPS AND FOUND THEM TO BE CLEAN, THEY ARE CLEAN WHILE THE THIRD ONE IS UNCLEAN; AND IF THEY EXAMINED THE THREE AND THEY WERE FOUND TO BE CLEAN, THEY ARE ALL UNCLEAN; SO R. MEIR, FOR R. MEIR RULED: ANY OBJECT THAT IS IN A PRESUMPTIVE STATE OF UNCLEANNESS ALWAYS REMAINS UNCLEAN UNTIL IT IS KNOWN TO YOU WHERE THE UNCLEANNESS IS. BUT THE SAGES RULED: ONE CONTINUES THE EXAMINATION OF THE HEAP UNTIL ONE REACHES BEDROCK OR VIRGIN SOIL.<span class="x" onmousemove="('comment',' And if no uncleanness can be found even there, it may be presumed that the heap is clean. ');"><sup>13</sup></span>