Brachot 29 - Eighteen blessings
Initially the written Torah was not allowed to be recited from memory, and the oral laws were not to be committed to writing. However, it was always known that eventually the phrase " The time has come to act for God " would allow to write down the prayers, for example, and this did happen in the time of Rabbi Yehudah the Prince.
When the prayers could only be recited by heart, Rabban Gamliel held that one should say the complete eighteen blessings of the Standing (Amidah) prayer every day, Rabbi Yehoshua - that one has to say the abridged prayer, and Rabbi Akiva - that it depends on one's proficiency. Even now, when one is in danger and is unable to concentrate, he can say the short prayer of Rabbi Yehoshua.
Why are there precisely eighteen blessings? - This corresponds to the eighteen praises of God in the psalm of David , some say, to the eighteen mentions of God in the Shema , and others - to the eighteen vertebrae in the spine, which bends in prayer. Later, another blessing, directed against the persecutors of the Jews, was inserted, but the prayer retained it name, "Eighteen blessings." But the number eighteen had a basis! There is yet another name of God in this psalm, another name ("One") in Shema, and another small vertebra.
If one makes his prayer a fixed obligation, it is not acceptable, that is, the prayer should not be a burden to him; some say, it should not be said at a fixed time of the day, but rather with the sunrise and sunset; others say - it should always be a supplication.
Art: Albert Anker - Boy Writing