מַתְנִי׳ אֵין מוֹדְדִין אֶלָּא בְּחֶבֶל שֶׁל חֲמִשִּׁים אַמָּה, לֹא פָּחוֹת וְלֹא יוֹתֵר. וְלֹא יִמְדּוֹד אֶלָּא כְּנֶגֶד לִבּוֹ.
MISHNA: One may measure a Shabbat limit only with a rope fifty cubits long, no less and no more, as will be explained in the Gemara. And one may measure the limit only at the level of one’s heart, i.e., whoever comes to measure the limit must hold the rope next to his chest.
Rashi Ibid
ולא ימדוד אלא כנגד לבו - קבעו לו חכמים מקום לשום כנגדו ראש החבל שלא יתן זה כנגד צוארו וזה כנגד רגליו והחבל מתקצר והתחומין מתמעטין
And will not measure except against his heart - the sages have set a standard for the terminal points of the rope, so that one will not measure from his neck and one from his legs, causing the distance to be shorter and the domain decreases.
The Gemara also explains that the length of the rope is 50 cubits, because a shorter rope will be too tight and cause the domain to be longer than is accurate, and a longer rope will hang to much and cause the domain to be shorter than is accurate.
I will indulge in a bit of “chassidish-style” drash on this section of the Gemara. (There is tremendous mystical potential lying within the legalistic details of Eruvin, see Maaseh Rokeach Seder Moed Maseches Eruvin on Mishna of Karpef
וה' יאיר עינינו בתורתו. וידעתי גם ידעתי שיש במסכתא זו רזין דרזין אך כבר כתבתי שאין לנו עסק בנסתרות, עד שיבא עני ורוכב על חמור ויתבערו רוח הטומאה מן הארץ ותמלא הארץ דיעה כמים לים מכסים. בבי"א.)
In any case, I cannot Help but notice that in these measurements we are told, that when one measures, one can only measure by their heart. Is this phrase an accident? After all, they could have chosen to measure from any part of the body; why not the head? Why not the waist? Furthermore, the choice of 50 cubits for the length of the Rope may not be arbitrary either. 50 is a significant number in Judaism. It is the number of days from Passover to Shavuous, which corresponds to the preparatory days leading from Redemption to the acceptance of the Torah. And it also, corresponds to The 50 Gates of wisdom as described in Rosh Hashana (21b). In addition the rope is not supposed to be too tight or too lose.
Is it possible that there is a deeper allegorical meaning here. That is, when one needs to set boundaries for oneself, if he or she wants to achieve it through wisdom, it needs to be made with the heart and not the head. The Mishna then is saying, literally and allegorically, "One may measure the boundaries only from the heart.”
for Video Shiur click here to listen: Psychology of the DAF Eruvin 57
Translations Courtesy of Sefaria
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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