The Primal Vow of Amida Buddha is inconceivable

 "Tathagata's Primal Vow surpasses conceptual understanding; it is a design of the wisdom of Buddhas. It is not the design of foolish beings. No one can fathom the wisdom of Buddhas, which surpasses conceptual understanding."[1]
 
"Further, with regard to Other Power, since it is inconceivable Buddha-wisdom, the attainment of supreme Enlightenment by foolish beings possessed of blind passions comes about through the working shared only by Buddhas; it is not in any way the design of the practicer. Thus, no working is true working. 'Working' that is negated refers to the calculation of the person of self-power. Concerning Other Power, then, no working is true working."[2] 
One who entrusts himself to Amida Buddha realizes that it is through Amida's Power that he is saved ("true working"), and not by his own designs and calculations or through the so called "merits" he thinks he has accumulated until now ("no working").
The fact that foolish beings are made to attain perfect Enlightenment after birth in the Pure Land, although they are now possesed by blind passions, is beyond conceptual understanding because, as Shinran said, it is related with the "inconceivable Buddha-wisdom" and it comes about "through the working shared only by Buddhas". This means that only Buddhas know the mechanism of Amida's salvation and only they can understand it. All we can do is to accept in faith that such foolish people like us, possesed of blind passions, will attain perfect Enlightenment once we enter the Pure Land of Amida.
We are like ignorant peasants who enter a plane for the first time. We do not know how the plane flies, but we trust the pilot and the engineers who built it, and we are certain that they will take us safely to the destination. 
 
The theme of the inconceivability of Amida Buddha's salvation is often mentioned in the sacred texts. Simply stated, unenlightened minds cannot understand the Enlightened Minds and work of Buddhas, just like an ignorant peasant who never got out of his village and who does not know how to read or write, cannot understand planes or the flying technology. Shinran Shonin even scolds Kyomyo-bo in a letter, saying that to try to understand with his limited mind or "conceptual understanding", the mechanism of the Vow and the Name of Amida, will only lead to confusion:
 
"Once you simply realize that the Vow surpasses conceptual understanding and with singleness of heart realize that the Name surpasses conceptual understanding and pronounce it, why should you labor in your own calculation?
It seems to me that with all your attempts to understand by reasoning and by learning you have fallen into confusion. It is completely in error. Once you have simply come to realize that Vow and Name surpass conceptual understanding, you should not calculate in this way or that. There must be nothing of your calculation in the act that leads to Birth.
You must simply entrust yourself to Tathagata."[3]
It seems to me that with all your attempts to understand by reasoning and by learning you have fallen into confusion. It is completely in error. Once you have simply come to realize that Vow and Name surpass conceptual understanding, you should not calculate in this way or that. There must be nothing of your calculation in the act that leads to Birth.
You must simply entrust yourself to Tathagata."[3]
 
We can easily read and understand the Primal Vow, which is the call of Amida Buddha to us, because it’s so simple that even ignorant peasants can get its message - "entrust yourself to me, say my Name and wish to be born in my land". We can say the Name of Amida, expressing our faith in Him, because it’s so easy to say it: "Namo Amida Bu" or "Namo Amitabha", "Namo Adidaphat" or "Namo Amituofo", etc.  But we cannot understand the exact supramundane details or mechanism through which Amida Buddha makes possible our birth in the Pure Land, just like the ignorant peasant does not know anything about flying technology.
 
As Shinran said, "pronounce" the Name even if its working "surpasses conceptual understanding" and entrust to Amida Tathagata even if He too, is beyond your capacity to understand: "you must simply entrust yourself to Tathagata".
 
Another reason why we cannot conceive Amida Buddha's salvation is because we are bound by ideas of merit and punishment. We do not have enough love for ourselves, not to mention other beings, so we might think that we must become worthy of such a grandiose birth in the Pure Land. However, to hear that "foolish beings possessed of blind passions" can "attain birth in the Pure Land" no matter they do not deserve it, and that they will "realize Nirvana" there, although they did not "severed their blind passions" during this life time, comes as a shock for our ego-centered minds focused on rewards and punishment:
 
"Why is this inconceivable? When foolish beings possessed of blind passions attain birth in the Pure Land, they are not bound by the karmic fetters of the three realms. That is, without severing blind passions, they realize Nirvana itself. How can this be conceived?"[4]
 
Truly, how can we conceive that somebody, namely Amida Buddha, can have such an unlimited and non-discriminatory Compassion that He devised a plan and a karmic mechanism to bring into His Pure Land such "foolish beings possessed of blind passions" like ourselves? But what we cannot conceive, and we cannot do ourselves for us and others, a Buddha can certainly do. So, we must simply let go to our calculating mind and entrust ourselves to Amida Tathagata.
 
"The Pure Land teaching is the inconceivable Dharma-teaching."[5], and certainly, the most difficult thing in the world to accept in faith, but also the shortest way to Nirvana for all beings, especially the lowest of the low:
 
"It is the Dharma by which ordinary and foolish beings bound by evil passions, those in the lower levels of society, such as hunters and traders, can instantly transcend birth and death and attain Buddhahood. This is called 'the Dharma which is the most difficult thing in the world to accept in faith.'”[6]

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The following video is on the same topic and much more. I hope you'll find it useful,

 

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[1] Shinran Shonin, Lamp for the Latter-Ages, letter 7, The Collected Works of Shinran, Shin Buddhism Translation Series, Jodo Shinshu Hongwanji-ha, Kyoto, 1997, p.533
[2] Shinran Shonin, A Collection of Letters, Letter 10, The Collected Works of Shinran, Shin Buddhism Translation Series, Jodo Shinshu Hongwanji-ha, Kyoto, 1997, p.571-572
[3] Shinran Shonin, Lamp for the Latter-Ages, letter 9, The Collected Works of Shinran, Shin Buddhism Translation Series, Jodo Shinshu Hongwanji-ha, Kyoto, 1997, p.536
[4] Shinran Shonin, A Collection of Passages on the Types of Birth in the Three Pure Land Sutras , The Collected Works of Shinran, Shin Buddhism Translation Series, Jodo Shinshu Hongwanji-ha, Kyoto, 1997, p.643
[5] Shinran Shonin, Lamp for the Latter Ages, letter 8. The Collected Works of Shinran, Shin Buddhism Translation Series, Jodo Shinshu Hongwanji-ha, Kyoto, 1997, p.535
[6] Shinran Shonin quoted Master Yuan-chao in Kyogyoshinsho – On Teaching, Practice, Faith, and Enlightenment, translated by Hisao Inagaki, Numata Center for Buddhist Translation and Research, Kyoto, 2003, p. 117