The Torah obligation to pray does not include a fixed text. At the beginning of the Second Temple period, the Men of the Great Assembly established the formalized text of the prayer service that remains in use. Their goal was to make it easier to concentrate during prayer and to create a unified text for the entire nation. The prayer service was formulated with great precision, and much thought was put into both its themes and its wording.

From the days of Moses until the days of the Men of the Great Assembly, there was no uniform structure of prayer. Instead, each individual composed his own prayers according to his understanding and wisdom. It was so until the Men of the Great Assembly came and instituted this prayer, the Shemoneh Esreh, so that it would be formulated for all. They formulated it in simple, understandable language, so that the ideas therein would not be confusing due to the language and it would be uniform for all, both the wise and the simple-minded. Nevertheless, they formulated it with great intent and extreme care, in that it is arranged with three blessings of praise before the petitionary prayers, and three following them. In between are twelve blessings that include petitions for all of a person’s needs. All of this was not just by chance, but done after long consideration. (Rabbeinu Baĥya ibn Ĥalawa, Commentary on the Torah, Deuteronomy 11:13)

Prayer is service of the heart, and requires mental exertion – both in keeping extraneous thoughts at bay and in concentrating on the words of the prayer. In addition, the words of the prayer have mystical meanings, which are the soul of the prayers. Praying in this mystical manner involves meditating on the names of God that appear in the text of the prayers.

Thought during prayer:

A person must perform service of the heart with great dedication. He must be as mighty as a lion to be alert that no extraneous thought enters his mind. If he is not able to empty his mind of extraneous thoughts during prayer, he should seek out techniques and strategies [to avoid them]. For instance, he should think: The Creator, may He be blessed, is standing opposite me, hearing what comes out of my lips, and checking the thoughts of my heart. He will then tremble and be full of shuddering and agitation. He should employ whatever resources he can think of that will enable him to overcome and remove his foreign thoughts. After that, he should be as mighty as a lion in another way, contemplating the meaning of the words. He should pour forth his words and spirit before the King of all kings, the Holy One, blessed be He, who hears prayers and can do the impossible. In truth, doing so takes great effort…. This is the explanation of why our Sages, of blessed memory, called prayer “service of the heart” (Mekhilta deRashbi 23:25), for it is great work, holy service.

Intention is the soul of prayer:

The matter of prayer with intention, which is the soul of prayer, is the mystical aspect of prayer. For the revealed aspect of prayer is that it is about temporal life. Similarly, recounting the praises of the Holy One, blessed be He, and how He, may He be blessed, directs his acts, are revealed matters. But their inner and mystical element, that which touches on the loftiest matters in the world, is the soul of the words. This element is the mystical meaning of the divine names, and the names are the sefirot through which the Lord is unified and through which the inner aspect of His unity, may He be blessed, is expressed. The Holy One, blessed be He, longs for this type of prayer…. The revealed aspect of prayer is analogous to the body and intention in prayer to the soul. (Rabbi Yeshaya Horowitz, Shenei Luĥot HaBerit, Masekhet Tamid, Ner Mitzva 10)

A person should turn to God in his own language and converse with Him about whatever happens in his life. This type of prayer has the advantage of not being “recognized” by any negative spiritual forces that are trying to destroy prayer. Nevertheless, a person must continue to pray according to the fixed prayer text established by the Sages and not limit himself to improvised and spontaneous prayer.

Prayers, supplications, and requests that have already been formulated by the Sages are known to all of the destructive spiritual forces and accusers. They ambush these types of prayers because they already recognize them. An analogy: On a paved highway, well known to all, murderers and bandits always lie in ambush because it is a road they already know about. But one who travels by a new and unknown path does not need to worry about murderers and bandits, for they do not know it as a place where they can lie in ambush. Similarly, a conversation that a person has between himself and his Maker is a new path, a new prayer that one says from his heart in a new way, so the accusers are not lying there in ambush for him. Nevertheless, he warned about how important it is to also say the other fixed supplications and requests. (Rabbi Naĥman of Breslov, Likutei Moharan 2:97)

There are prayers that flow from the heart and break out of the depths, and at other times it is the mind that arouses the heart to pray. For standard prayers, whose texts are fixed, inspiration generally stems from the mind, whereas prayers that spring forth from a person’s heart are usually connected to personal experiences and have no fixed and ordered text.

Prayer [can come about] in two ways: Sometimes the heart is inspired to pray, as the verse states: “Their hearts screamed out to God” (Lamentations 2:18). As a result of one’s troubles in his heart, prayer breaks forth from its deepest recesses. Prayer can also come about through thought and meditation, inspiring one to pray. Even though prayer is impossible without both mind and heart, nevertheless one type of prayer emerges because of the distress he feels in his heart and that inspires his mind to think thoughts of prayer, whereas the other emerges through meditating with his mind, and the mind arouses the heart – which is the soul – to pour forth to its source…. We can say that this is also the difference between the fixed prayers instituted for us by the Men of the Great Assembly and the prayers that individuals say because of the events that occur on a particular day. (Rabbi Shmuel Bornsztain, Shem MiShmuel, Va’etĥanan 5675)