Rabbi Akiva in Avos (3:18) teaches, "chaviv adam shenivra b'tzelem - beloved is man for he was created in G-d's image". Rashi (Braishis 1:26) understands this to mean l'havin u'l'haskil - to understand and to gain wisdom. Man's intelligence is a reflection of his being created in G-d's image, and is the yardstick by which his actions are measured. Does he utilize the Divine gift of intelligence properly or not? Thus, the generation of the flood was annihilated as they went against their sechel (intelligence) with their rampant chamas (robbery). (In contrast, notes Rashi (Braishis 11:9), the dor haflaga, the generation that attempted to build the tower, was built upon love and friendship.)
The above cited mishna continues, "chavivin Yisroel shenikre'u banim laMakom - beloved are the people Israel, for they are described as children of Hashem." The mishna is providing for us a progression. First one has to be an adam - a mentsch, and only then he can be included among the banim. The Torah of the banim, i.e. the mitzvos, is clearly stated beginning in Shemos (chapter 12). What however is toras adam? This is sefer Braishis, and specifically the personal lives, examples, and lessons we learn from the avos.
The Gemara (Avodah Zarah 25a) identifies sefer Braishis as "Sefer Hayashar - the book of the upright", as it describes the lives of the avos who are called yesharim. In parshas Lech L'cha (13:3) the Torah teaches "vayelech l'masa'av" describing Avraham's travels. On his return trip from Mitzrayim, he stayed in the same lodgings he had stayed in on his way to Egypt. Rashi says- "Lelamdecha derech eretz"- Avraham was teaching proper conduct. One should not change his lodgings when traveling the same route, lest it be taken as a sign of dissatisfaction with the first host, and hence an affront to his dignity. This is part of toras adam. In addition- who told Avraham to endanger his life and rescue Lot? The Ramban (Bereishis 19:29) attributes this to Avraham's hakaras hatov, deep-felt appreciation of Lot's accompanying him. Once again, this is Avraham using his G-d given gift of intelligence- toras adam.
Chovos Halevavos teaches that Hashem communicates to us through sechel (intelligence). It is a holy directive from on High. Thus, there is a complete chapter (O.C. 170) in Shulchan Aruch dedicated to standards of propriety in accordance with which a person should conduct himself during his meal. For example, one may not to eat as a glutton. One should sit and eat slowly. These are parts of toras adam. It is wrong to ask- where does it say that one is to conduct himself in accordance with the above? The answer is chaviv adam- this defines how an adam lives.
The Kotzker Rebbe zt"l noted on the verse, "veanshei kodesh tiheyu li - people of holiness shall you be to me" (Shmos 22:30), that the Torah first mandates that we be anshei- a person, a mensch, and the build upon that to become a kadosh (holy).
Too often the kiddush following davening does not reflect this toras adam. Similarly, the environment and demeanor at the shmorgassbord too often can bring out the worst in us. Chaviv adam is a directive to all mankind, and unfortunately too often we neglect it, substituting pleasure and technology in its stead. Compare photographs of people 50 years ago to that of this day and one can see the failing of a lack of toras adam.
Koheles (7:29) teaches, G-d has made man yashar (straight), but man has sought out many intrigues. Hashem made man a yashar by endowing him with sechel (intelligence). This is the book of Breishis and the many lessons of the avos are chapters in yashrus. The too familiar excuse, "it doesn't say in the Mishna Brurah" or "show me where it says it's assur", and then concluding that it therefore must be muttar, is an affront upon toras adam. May we be privileged to actualize the great potential of tzelem Elokim that Hashem has invested in us.