Piranesi's amnesia makes him an unreliable narrator. While he seems to be a master of objective observation, his perception of reality is skewed by his inability to remember his own identity and the existence of the outside world. Take, for example, this journal entry titled "Do trees exist?" In it, Piranesi speculates on the origin of a leaf:
Many things are unknown. Once [...]I saw a bright yellow speck floating on a gentle Tide beneath the Fourth Western Hall. Not understanding what it could be, I waded out into the Waters and caught it. It was a leaf, very beautiful, with two sides curving to a point at each end. Of course it is possible that it was part of a type of sea vegetation that I have never seen, but I am doubtful. The texture seemed wrong. Its surface repelled Water, like something meant to live in Air.
The fact that Piranesi uses the word "tree" but then goes on to question where the leaf might have come from shows the gaps in his understanding of the world. Similarly, Piranesi's habitual lack of suspicion in interactions with the Other suggests that his naivety might hinder his understanding of reality. This unreliability builds narrative tension, since readers are often able to piece together truths about Piranesi's world before he does, contributing to the novel's intense dramatic irony.