From the Contemporary Poetry Review, here's an article with an interesting explication of Blake:
Although sometimes appearing to reject reason outright, Blake in his masterpiece of satirical irony The Marriage of Heaven and Hell tries to reconcile such apparent opposites as reason and energy, body and soul, through the office of the poetic imagination. His book also challenges conventional views of heaven and hell. Most remarkably, in the following section titled “A Memorable Fancy” (Plate 12), Blake addresses some of the objections of the Deists:
The Prophets Isaiah and Ezekiel dined with me, and I asked them how they dared so roundly to assert that God spake to them; and whether they did not think at the time, that they would be misunderstood, & so be the cause of imposition.
Isaiah answer'd. 'I saw no God, nor heard any, in a finite organical perception; but my senses discover'd the infinite in every thing, and as I was then perswaded, & remain confirm'd, that the voice of honest indignation is the voice of God, I cared not for consequences but wrote.'
Then I asked: 'does a firm perswasion that a thing is so, make it so?'
He replied: 'All poets believe that it does, & in ages of imagination this firm perswasion removed mountains; but many are not capable of a firm perswasion of any thing.'
Then Ezekiel said. 'The philosophy of the east taught the first principles of human perception: some nations held one principle for the origin & some another; we of Israel taught that the Poetic Genius (as you now call it) was the first principle and all the others merely derivative, which was the cause of our despising the Priests & Philosophers of other countries, and prophecying that all Gods would at last be proved to originate in ours & to be the tributaries of the Poetic Genius; it was this that our great poet King David desired so fervently & invokes so pathetic'ly, saying by this he conquers enemies & governs kingdoms; and we so loved our God. that we cursed in his name all the deities of surrounding nations, and asserted that they had rebelled; from these opinions the vulgar came to think that all nations would at last be subject to the jews.'
'This' said he, 'like all firm perswasions, is come to pass; for all nations believe the jews' code and worship the jews' god, and what greater subjection can be?'
I heard this with some wonder, & must confess my own conviction. After dinner I ask'd Isaiah to favour the world with his lost works; he said none of equal value was lost. Ezekiel said the same of his.
I also asked Isaiah what made him go naked and barefoot three years? he answer'd, 'the same that made our friend Diogenes the Grecian.'
I then asked Ezekiel why he eat dung, & lay so long on his right & left side? he answer'd, 'the desire of raising other men into a perception of the infinite; this the North American tribes practise, & is he honest who resists his genius or conscience. only for the sake of present ease or gratification?'
Though Blake’s irony is multi-layered, there’s a kernel of genuine insight within this fanciful tale. According to Blake, Isaiah’s revelations of God’s judgments were not bound by history or geography. The prophet said that he didn’t hear God “in a finite organical perception,” but rather was firmly persuaded that “the voice of honest indignation is the voice of God.” Since the voice of honest indignation can be heard by anyone throughout the globe, it is potentially available to all, answering the Deists’ demand for universality. Blake even makes provisions for the Indians in “worlds discovered new” when he compares the physical austerities practiced by Ezekiel to those of “the North American tribes,” suggesting that both practices are impelled by a universal desire to alter human consciousness to experience a “perception of the infinite.” Similarly, Blake has Isaiah assert that the impulse that made him go naked and barefoot for three years was shared by the Greek Cynic Diogenes.
According to this “Memorable Fancy,” the chief difference between the religion of Jews and Christians and those of other people is that the people of Israel taught that “the Poetic Genius” was the first principle, from which all others are derived. Similarly, the only difference between the descendants of the “great poet King David” and other nations is the greater emphasis the former place on the poetic imagination.