The Mansion of Many Apartments is a metaphor that the poet John Keats expressed in a letter to John Hamilton Reynolds dated Sunday, 3 May 1818.
Here Keats suggests that people were capable of different levels of thought. Those who did not consider the world around them remained in the thoughtless chamber. Even though the door to move on to the next "apartment" was open, they had no desire to think any deeper and to go into that next apartment.
When you did move on into the next chamber, you would for the first time have a choice of direction, as from this apartment there were several different dark passages. Keats believed that he was at this point when he wrote the letter, as was William Wordsworth when he wrote Tintern Abbey.
Keats expressed this idea in The Fall of Hyperion: A Dream (1819).