In his book The Romantics, Neil King defines Blake as part of the Romantic movement in the following way:
Blake was not interested in strict representational ‘correctness’ but was more concerned with bringing out imaginatively what an experience meant to him. In this Blake is characteristically Romantic, believing in the centrality of the imagination, and that an artist must reject the past and find his own way of doing things from within himself. He said, ‘I must create my own system or be enslaved by another man’s.’
Blake’s individual perspective on the universe, however, was in no way vague or flighty. G.K. Chesterton makes this clear in his biography of William Blake:
No man had harder dogmas; no-one insisted more that religion must have theology. The Everlasting Gospel [ie Blake’s own invented theology] was far from being a simple gospel. Blake had succeeded in inventing in the course of about ten years as tangled and interdependent a system of theology as the Catholic Church has accumulated in two thousand. Much of it, indeed, he inherited from ancient heretics who were much more doctrinal than the orthodoxy which they opposed.
Blake therefore avoided the refuge of relativism that often accompanies the absence of a transcendent referent (such as Christianity or Marxism), by creating his own fully worked-out theology. As Chesterton says:
Blake was one of the most consistent men that ever lived, both in theory and practice. Blake may have been quite wrong, but he was not in the least unreasonable.
He remarks on Blake’s character:
You might call him a solid maniac or a solid liar; but you could not possibly call him a wavering hysteric or a weak dabbler in doubtful things.