Reference for Menachot 69:2
א"ל רבא מאי שנא גוייתא לברייתא וברייתא לגוייתא דלא דהך דבעי למיחזי אוירא לא קא חזייא והא דלא קא בעי למיחזי אוירא קא חזייא ברייתא לברייתא וגוייתא לגוייתא נמי הך דבעיא למיחזי אוירא דימין קא חזיא אוירא דשמאל ודשמאל קא חזיא אוירא דימין אלא ל"ש
Thereupon Raba said to him, Why is it that [the placing of] an inside portion outside or of an outside portion inside is not valid? It is, is it not, because that which should look out into the open does not do so, whilst that which should not look out into the open actually does so? Then, likewise, [the placing of] an outside portion also outside or an inside portion also inside [should also be invalid], since what should look out into the open on the right looks out on the left, and what should look out into the open on the left looks out on the right?