Reference for Eruvin 27:18

עגולה רואין אותה כאילו היא מרובעת: הא תו למה לי סיפא איצטריכא ליה כל שיש בהיקיפו ג' טפחים יש בו רחב טפח

But, then, is not this exactly the same [principle as was already enunciated]?<span class="x" onmousemove="('comment',' In the previous clause: (THE BEAM IS VALID) . . ALTHOUGH IT IS NOT STRONG. One that 'WAS MADE OF STRAW' is obviously not strong.');">¹⁶</span> - It might have been assumed that [the principle] is applied only to one of its own kind<span class="x" onmousemove="('comment',' Sc. a frail beam of wood may be regarded as a strong beam of the same material, since weak as well as strong beams can be made of it.');">¹⁷</span>

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