The most important part of Blake's poetical work is formed by the songs of innocence and the songs of experience. The first dealing with childhood as the symbol of innocence, a state of the soul connected with happiness, freedom and imagination. The collection contains poems written in a simple, musical language, rich in symbols drown from the Bible and Christian pastorals. Instead in the second there are a more pessimistic view of life emerges in the powerful symbolism used by Blake. These songs are intended to be read together with the songs of innocence so that the paired poems comment on each other. This aspect introduces one of the most important characteristic of Blake's work: complementary opposites. Indeed he thought that "without Contraries there is no Progression. Attraction and Repulsion, Reason and Energy, Love and Hate are necessary to human existence". The possibility of progress, of achieving the knowledge of what we are, lies the tension between opposite states of mind, not in their resolution by one gaining supremacy over the other. The two states coexist not only in the human being but also in the figure of the Creator who can be at the same time a God of love and innocence and the God of energy and violence. To understand Blake's thinking it is important his relationship with Christianity which is not ascetic, liturgical or moralistic. He believed in the reality of a spiritual world but regarded Christianity, and the Church especially, as the responsible for the fragmentation of consciousness and the dualism characterised man's life. So he was interesting only in the role of the spiritualism which is played by imagination, which was considered by Blake as the means through which Man could know the world. God, the child nd the poet share this power of creating things. The poet therefore becomes a sort of prophet who can see more deeply into reality and who also tries to warn man of the evils of society. Indeed Blake was actually concerned with political and social problems of his time: he supported the abolition of slavery and shared other intellectuals' enthusiasm for the egualitarian principles. In his poems he sympathised with the victims of oppression by institution such as orphan, prostitutes and soldiers.
Blake's art had many aspects of pre-romanticism as the attention to the lower class and the interesting in denouncing social problems. Indeed Blake wasn't like his contemporary and his poem was very different with epic poems which were deal to aristocracy. He introduced a simple language which is musical and rich in symbols. He also developed the theme of imagination and introduced, with the idea of complementary opposite, one of the most important characteristic element of romanticism: the sublime.
It is the Divine Vision and it is associated with the idea of God. It is also the collation of emotions which become from the soul; so it is considered by Blake as the means through which man could know the world, could see more, beyond material reality, into the life of things. Blake thought that man had lost the possibility of using imagination because of many restrictions imposed by reason and society. Poets, and artists in general, can reconcile innocence, imagination with experience so they became prophet. Indeed poet is the mediator who can see more deeply into reality so he can help the people to identify and to understand the evils of society.
In the choice of his themes Blake anticipated the grater Romantic poets just as he did in his mode of expression. His poems present a very simple structure and highly individual use of symbols. With the analysis of symbols with can have a deep approach which is necessary to understand Blake's ideas. There are some recurrent symbols like, for example, the lamb which represents child and Christ and so the innocence.