Derekh Hashem, Part Four, On the Daily Order of Prayer

The sections of prayer: However the whole of prayer is divided into four sections: The first is the sacrifices; the second is the verses of song (pesukei dezimrah); the third is the recitation of Shema and its blessings; and the fourth is [the actual] prayer and that which follows it. The general intention of the sacrifices is to purify all the world, and to remove anything from it which would be an interference or a prevention for the coming of the supreme flow down to it. The general intention of the verses of song is to reveal the light of His presence, may He be blessed, through the praises with which we praise Him and recount His acclaim. For the Creator, may He be blessed, made it depend on this act, meaning praising Him. And this is the [meaning] of "the One who chooses the verses of song" (at the end of this section). We have already explained the general content of the recitation of Shema and its blessings. But besides that, there is another matter. And that is that which we have already explained in Part I (On the Spiritual Realm 3), that the orders of the creation's setup and its unfolding is such that - besides all of the creatures unfolding one level after another from the root forces to the physical ones - the Highest Wisdom set up that since the creatures all receive flow from in front of Him, may He be blessed, they first be connected one to another from the bottom up. [So] the bottom ones [are connected] to the ones above them and the higher ones to the ones above them and so on, in this way, until the root forces; and they [in turn] depend upon Him, may be blessed - such that He flows upon them [first] and they then spread the flow from above to below through all of the levels of the creation, as is appropriate. They then come back and all stand upon their levels for their tasks, according to that which is arranged for them. Moreover, these blessings over the recitation of Shema are ordered according to these secrets. So with this acclaim and praise, the levels of creation rise from below to above until they are all bound to the highest level. And then everything is bound and dependent upon His light, may He be blessed; and His flow comes down to all of the creatures, which is that which is done in the prayer of the eighteen (shmoneh esreh) [blessings].