"Then dwell in Possibility— / A fairer house than Prose—"
In this metaphor, the poet compares "Possibility" to a "house" that is "fairer" than "Prose". This suggests that the realm of possibility is more beautiful, inspiring, and imaginative than the realm of practical, everyday reality represented by "Prose".
The poet encourages the reader to dwell in this realm of possibility where anything is possible and imagination can soar. This metaphor captures the essence of the poem, which celebrates the power of imagination and the beauty of embracing the potential for endless possibilities in life.