Your inquiry is a valuable question because it calls forth an explanation of a little known phenomenon in the evolution of liturgical blessings during the formative period of Rabbinic Judaism, from 200 B.C.E. to 200 C.E. The early prototype of the shehechiyanu blessing had only four words, "Barukh shehigiyanu lazman hazeh". It was simply an expression of gratitude that "He enabled us to reach this moment in time". This is the formula found in the Kaufmann manuscript which preserves the early versionof MishnahBerakhot 9,3 where building a house and acquiring new garments are mentioned. This is the wording in Talmud Yerushalmi, Berakhot, 9, 4 for new garments (quoted in the name of Rabbi Aha) and in Tosefta Berakhot ch. 6, 14 for building a new sukkah, and 6,15 for producing one's own tefillin,lulav or tzitzit.
A working assumption in liturgical research is that Jewish prayers first developed in popular culture, folk custom and prayer assemblies, and later were standardized. After the destruction of the 2nd Temple and the devastating blows of exile, rabbinical leaders began shaping ritual by encasing it with meaning and ensconcing observance in liturgical form. Blessings were standardized with the specific purpose of focusing thoughts and emotions.
Thus, the original 4 words were expanded into 12, and the blessing became known as Shehechiyanu. It was now applied to a variety of new situations – eating new fruits in their seasonal appearance, marking the annual arrival of festivals and ritual observances. Shehechiyanu was incorporated into the calendar of events for mitzvot such as blowing shofar, reading megilah, sitting in a sukkah, eating matzah and lighting Hannukah candles. The blessing came to include the full blessing formula with God's name and two new verbs. I suggest explaining the three key expressions as follows:
1.Shehechiyanu - That He has given us life, hinting at the primal form of birth.
2.Vikimanu And He is sustaining us, implying a continued existence, nourishment and perseverance over time.
3.Vehigiyanulazman hazeh - That He has brought us to this time, to the specific present, the special moment in time.
Some medieval commentators have observed that each of the three verbs expresses a different conceptualization of gratitude. R. Eleazar Rokeach (1176 – 1238), in his commentary on the prayer book, HaRokeach, 361, explains that the three verbs parallel three distinct forms of praise and gratitude in the realms of soul, life, and essence as hinted at in Psalms 146, 1-2.
In conclusion, the Shehechiyanu blessing in its current form was instituted to fulfill multiple purposes that are not covered by other blessings. Shehechiyanu can be said to offer an opportunity for a heightened awareness of life, time and existence. In that sense, it is the most meditative of halachic blessings in that it focuses attention on the present moment in time, cultivates an appreciation of the fleeting presence and expresses recognition of a unique achievement. For those of us who have a meditative-prayer inclination, Shehechiyanu might be perceived today as a Jewish alternative to the popular Buddhist Vipassanā mindfulness practices. For those interested in more explanations, I have published a booklet on this subject and it is available from Natan@JewishMeditation.org.il
 On the development of normative prayer liturgy see the classical book by Joseph Heinemann, Prayer in the Period of the Tannai'm and the Amora'im : Its Nature and Its Patterns, Jerusalem, 1978. For more recent scholarly research see the website of Dr. Tzvee Zahavy, http://www.tzvee.com/Home, and in particular, his article on the Politics of Piety http://www.tzvee.com/Home/the-politics-of-piety, and Studies in Jewish Prayer, http://www.zahavy.com/studies.html
The first exegetical attempt at explaining the Biblical roots of the Shehechiyanu blessing can be found in the 12th century midrashic compilation, Midrash Shekhel Tov (Genesis 22 and Exodus 12), where Shehechiyanu – "He has given us life", is derived from Deuteronomy 4,4, "You are clinging (hadevekim) to Hashem your God and thus you are Alive today".
The Shehecheyanu blessing or berakhah is recited when we do something infrequently or upon special occasions. It originates in the Talmud and is referenced in several places, for example: Pesachim 7b; Sukkah 46a; and Berakhot 37b, 44a, and 59a. Like most blessings, the Shehecheyanu was introduced by the early Rabbis and part of our Rabbinic tradition.
The Shehecheynau reads, "Blessed are You, Lord our God, Ruler of the Universe, Who has kept us alive (shehecheyanu), sustained us and brought us to this moment in time." It is recited at the start of a festival, eating a fruit of a new season for the firts time, performing mitzvot such as taking the lulav, hearing the shofar, reuniting with an old friend, acquiring a new home, new household item, or clothing, as well as other places (e.g., festival kiddush, Kol Nidre). Furthermore, different communities have particular customs pertaining to the inclusion of the Shehecheyanu for additional special occasions.
An early mention of Shehecheyanu appears in Tractate Berachot of the Mishnah, the first generation of commentary on the sacred obligations found in Torah. In Berachot 9:3, the Rabbis instruct us to recite this prayer over the purchase of a new house or when acquiring new vessels. The citation of the Shehecheyanu here is anonymous, which tells us two things: one, that we cannot ascertain its authorship; and two, that it was the accepted procedure at that time. There seems to have been some dispute about when it was appropriate to say it, but including it in our lexicon of prayers was mandatory.
The Mishnah was composed very early in the history of the Rabbinic sources: It began as an oral tradition, and was only written down upon the permission – some might say insistence – of Rabbi Yehudah Hanasi (Rabbi Judah the Prince) in or about the year 200 CE. Therefore we have to assume that this benediction (“berachah” in English) is at least from the Second century if not earlier. We do not know its precise origins, but this seems to be the earliest mention of it in tradition.
(Origins of this prayer might hearken back to Deuteronomy 8:10: “When you have eaten and are sated, then you will bless the Eternal your God for the good land which [God] has given you,” as this is a reminder of the gratitude that we should express to God for all God’s beneficence. But it is uncertain whether this or any particular Tanakh quote is the inspiration for Shehecheyanu.)
There is much discussion - and natually some disagreement - within the sources about when and under what circumstances we say "Shehecheyanu.” That is, we might say this blessing when acquiring new property, at a holiday that we observe annually, when we perform certain sacred obligations (mitzvot), or even at special and unique moments in our lives that occur only very occasionally. But as to its dating, it appears first in the Mishnah, as mentioned above.
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