כל העוסק [בשלשתן] במקרא במשנה ובדרך ארץ על זה נאמר (קוהלת ד) והחוט המשולש לא במהרה ינתק.
The verse, "A cord made of three strands is not readily broken" (Ecclesiastes 4:12) refers to someone involved in three things: The Written Torah, the Oral Torah, and Menschlich living [Derech Eretz].
Another explanation: And you shall make the boards for the tabernacle of acacia wood standing up. Why of acacia wood? God was teaching Menschlichkeit for the generations, that when a man is about to build his house from a fruit-producing tree, he shoud be reminded: If, when the supreme King of Kings commanded the Temple to be erected, His instructions were to use only such trees as are not fruit-bearing--though all things belong to Him--how much more should this be so in your case?
מַעֲשֶׂה בְּרַבִּי יַנַּאי שֶׁהָיָה מְהַלֵּךְ בַּדֶּרֶךְ וְרָאָה אָדָם אֶחָד שֶׁהָיָה מְשֻׁפַּע בְּיוֹתֵר, אֲמַר לֵיהּ מַשְׁגַּח רַבִּי מִתְקַבְּלָא גַבָּן, אֲמַר לוֹ אִין, הִכְנִיסוֹ לְבֵיתוֹ הֶאֱכִילוֹ וְהִשְׁקָהוּ, בְּדָקוֹ בְּמִקְרָא וְלֹא מְצָאוֹ, בְּמִשְׁנָה וְלֹא מְצָאוֹ, בְּאַגָּדָה וְלֹא מְצָאוֹ, בְּתַלְמוּד וְלֹא מְצָאוֹ, אֲמַר לֵיהּ סַב בְּרִיךְ, אֲמַר לֵיהּ יְבָרֵךְ יַנַּאי בְּבֵיתֵיהּ, אֲמַר לֵיהּ אִית בָּךְ אֲמַר מַה דַּאֲנָא אֲמַר לָךְ, אֲמַר לֵיהּ אִין, אֲמַר לֵיהּ אֱמֹר אָכוֹל כַּלְבָּא פִּיסְתְּיָא דְּיַנַּאי, קָם תַּפְסֵיהּ אֲמַר לֵיהּ יְרוּתָתִי גַבָּךְ דְּאַתְּ מוֹנֵעַ לִי, אֲמַר לֵיהּ וּמַה יַרְתּוּתָךְ גַבִּי, אֲמַר לֵיהּ חַד זְמַן הֲוֵינָא עָבַר קַמֵּי בֵּית סִפְרָא, וּשְׁמָעִית קָלְהוֹן דְּמֵנִיקַיָא אָמְרִין (דברים לג, ד): תּוֹרָה צִוָּה לָנוּ משֶׁה מוֹרָשָׁה קְהִלַּת יַעֲקֹב, מוֹרָשָׁה קְהִלַּת יַנַּאי אֵין כְּתִיב כָּאן אֶלָּא קְהִלַּת יַעֲקֹב. אֲמַר לֵיהּ לָמָּה זָכִיתָ לְמֵיכְלָא עַל פְּתוֹרִי, אֲמַר לוֹ מִיּוֹמַי לָא שְׁמָעִית מִילָא בִּישָׁא וְחִזַּרְתִּי לְמָרַהּ, וְלָא חָמֵית תְּרֵין דְּמִתְכַּתְּשִׁין דֵּין עִם דֵּין וְלָא יְהַבִית שְׁלָמָא בֵּינֵיהוֹן. אֲמַר לֵיהּ כָּל הֲדָא דֶּרֶךְ אֶרֶץ גַּבָּךְ וְקָרִיתָךְ כַּלְבָּא, קָרָא עֲלֵיהּ שָׁם דֶּרֶךְ, דְּשָׁיֵם אָרְחֵיהּ סַגֵּי שָׁוֵי, דְּאָמַר רַבִּי יִשְׁמָעֵאל בַּר רַב נַחְמָן עֶשְׂרִים וְשִׁשָּׁה דוֹרוֹת קָדְמָה דֶּרֶךְ אֶרֶץ אֶת הַתּוֹרָה, הֲדָא הוּא דִכְתִיב (בראשית ג, כד): לִשְׁמֹר אֶת דֶּרֶךְ עֵץ הַחַיִּים, דֶּרֶךְ, זוֹ דֶּרֶךְ אֶרֶץ, וְאַחַר כָּךְ עֵץ הַחַיִּים, זוֹ תּוֹרָה.
Rabbi Yannai was once walking along the road, and saw a man who was extremely well dressed. Rabbi Yannai said to him: Would you like to come over to my house? The man replied: Yes. Rabbi Yannai brought him into his home, and gave him food and drink. As they were eating and drinking together, he examined him in his knowledge of Bible, and found out that he had none; examined his knowledge of Mishnah, and realized that he had none; his knowledge of legends, and saw that he had none; his knowledge of Talmud and saw he had none. Rabbi Yannai then told him: Wash and recite grace. Said the guest: Let Yannai recite grace in his own home. Seeing that he could not even recite a blessing, Yannai told him: Can you at least repeat what I say? Said he: Yes. Said Rabbi Yannai: repeat the following: 'A dog has eaten Yannai's bread.' Offended, the man stood up, and grabbed Rabbi Yannai by the coat! He then said: My inheritance is with you, and you are withholding it from me! Said Rabbi Yannai with puzzlement: What legacy of yours is there with me? He replied: Once I passed by a school, and I heard the voices of the little children saying: 'Moses gave us the Torah, the inheritance of the congregation of Jacob.' They did not say 'the inheritance of the congregation of Yannai,' but the 'congregation of Jacob.' Rabbi Yannai asked, “How then are you worthy to eat at my table?” The guest replied, “Never have I heard an evil word spoken against me and returned to argue with the person who spoke it. Never have I seen two people arguing without making peace between them.” Rabbi Yannai then said, “you have so much Derech Eretz and I called you a dog.” On him rabbi Yanai said the verse “And there is a path” – meaning not one, but two paths take you to salvation – since rabbi Ishmael son of rav Nachman said: Derech eretz precedes Torah by 26 generations, since it is written “and to guard the way to the Tree of Life” (Genesis 3). “Way” is the derech eretz, and only after that comes “Tree of Life” which is Torah.
Rabbi Samson Raphael Hirsch
The spirit of Israel is in with all culture, provided it leads towards the recognition of truth and well-doing. If, however, a culture is enslaved to sensuality, it will only lead to further corruption...Where there is no Torah, there is no Derech-Eretz; if culture does not pave the way for Torah, but strives towards its replacement, it does not lead the way to the Tree of Life, but to corruption.