Blake was very conversant with what the Bible has to say about Good and Evil:
Gen 1:31 "God saw all that he had made, and it was very good. And there was evening, and there was morning—the sixth day." It seems that everything was very good; there's no polarity here.;
But in Gen 2:8-9 we come to a complication:
"And the LORD God planted a garden eastward in Eden; and there he put the man whom he had formed.
And out of the ground made the LORD God to grow every tree that is pleasant to the sight, and good for food; the tree of life also in the midst of the garden, and the tree of knowledge of good and evil."
This seems to infer that Good and Evil came into existence as a consequence of the (biblical) Fall. In the pristine Garden before Adam and Eve's fatal mistake Evil had not entered the picture. (Some Bible scholars have concluded that the 'fatal mistake' was a culpa felix (Augustine, Aquinas, Ambrose). However it's generally understood as the cause of all unhappiness in our poor World. Imagine how it would be if the 'fatal mistake' had not occurred. Would we be more like the animals? or the angels?
With The Marriage of Heaven and Hell Blake put an entirely new slant on the subject. (The cavalier way Blake used the biblical Fall here illustrates the use Blake put to the Bible in general: like any other document everything was grist for his mill.) Speaking ironically he described Good as being sheeplike, and Evil as being active and creative. He described conventional people as the Elect, and active, creative people as Reprobate (btw he included Jesus among the Reprobates- following Isaiah 53:12).
The Elect were the angels in MHH; the Reprobates were the devils.
But Blake didn't stick to these definitions; MHH was the work of an angry young man. The mature Blake returned to more conventional meanings for 'angel' and 'devil'.
Returning to the Tree of the Knowledge of Good and Evil we may read in Blake's Songs of Innocence and of Experience:
Illustrations to Young's Night Thoughts
The Human Abstract.
Pity would be no more,
If we did not make somebody Poor:
And Mercy no more could be,
If all were as happy as we;
And mutual fear brings peace;
Till the selfish loves increase.
Then Cruelty knits a snare,
And spreads his baits with care.
He sits down with holy fears,
And waters the ground with tears:
Then Humility takes its root
Underneath his foot.
Soon spreads the dismal shade
Of Mystery over his head;
And the Catterpiller and Fly,
Feed on the Mystery.
And it bears the fruit of Deceit,
Ruddy and sweet to eat;
And the Raven his nest has made
In its thickest shade.
The Gods of the earth and sea,
Sought thro' Nature to find this Tree
But their search was all in vain:
There grows one in the Human Brain"
(Erdman p. 27)
We may see here the origin of the Tree of Mystery, which in Blake corresponds to the Tree of the Knowledge of Good and Evil; Blake has tried to explain the meaning of the Tree he had read about in Genesis.
Four Zoas, Night VII ,Page 81, (E 365) "Of all his wandering Experiments in the horrible Abyss He knew that weakness stretches out in breadth & length he knew That wisdom reaches high & deep & therefore he made Orc In Serpent form compelld stretch out & up the mysterious tree He sufferd him to Climb that he might draw all human forms Into submission to his will nor knew the dread result Los sat in showers of Urizen watching cold Enitharmon His broodings rush down to his feet producing Eggs that hatching Burst forth upon the winds above the tree of Mystery Enitharmon lay on his knees. Urizen tracd his Verses In the dark deep the dark tree grew. her shadow was drawn down Down to the roots it wept over Orc. the Shadow of Enitharmon Los saw her stretchd the image of death upon his witherd valleys Her Shadow went forth & returnd Now she was pale as Snow When the mountains & hills are coverd over & the paths of Men shut up But when her spirit returnd as ruddy as a morning when The ripe fruit blushes into joy in heavens eternal halls Sorrow shot thro him from his feet it shot up to his head Like a cold night that nips the root & shatters off the leaves Silent he stood oer Enitharmon watching her pale face He spoke not he was Silent till he felt the cold disease Then Los mournd on the dismal wind in his jealous lamentation"
Good and Evil are a polarity, and a contrary of the pristine oneness of the original Garden. We may see it as the first contrary, from which all others sprang. We live in a dualistic world, and people in general can only see things in black and white (like infants do). To perceive things as a spectrum, such as 'Good, less good, still less good,' etc. is a step away from the fatal tree, but still a long way from the primeval oneness from which we came and to which we are destined to return.