This is a curious poem that can often be hard to interpret in terms of its meaning. However, it appears that this poem traces the various stages of some kind of mental breakdown, using the imagery of a funeral to describe those stages.
The lyric begins by establishing the basic proposition of the poem: the speaker has had some kind of mental experience which is likened to the events of a funeral - a psychological death becomes merged with a physical death:
I felt a Funeral, in my Bran,
And Mourners to and fro
Kept treading - treading - till it seemd
That Sense was breaking through -
In the speaker's mind, the mourners seem to be walking up and down, perhaps paying their last respects to the dead person lying in the coffin. They are then seated and the funeral service begins. Once this stage of the service is complete, the coffin is lifted and taken to the graveyard while the church bell tolls. The coffin is placed on planks set over the grave and then one of the planks breaks and the coffin drops down into the grave:
And then a Plank in Reason, broke,
And I dropped down, and down -
And hit a World, at every plunge,
And Finished knowing - then -
Critics argue that this final stanza suggests the mind's final plunge into the abyss of despair or depression.
Although we are never told precisely what triggered these effects, it is clear that the funeral imagery serve to give concrete form to abstract feelings. The much-debated last stanza does obviously indicate some final descent, but some critics argue that this represents the complete loss of all thought, understanding and knowledge. Others argue that after breaking through the bounds of reason the speaker finishes by retaining or learning something, as indicated by the open dash. Either way, this poem traces some form of mental breakdown through using the stages of the funeral ritual to chart the gradual disintegration of the speaker's senses.