In this story, Hawthorne calls attention to the role of the narrator, who is revealed to be quite unreliable, leading the audience to question the veracity of the tale being told. Much of the language of the story highlights the act of storytelling. Rather than simply stating that Dr. Heidegger’s study is a strange place, the narrator qualifies the description of the setting with the phrase, “If all stories are true,” suggesting that the narrator isn't even certain what, exactly, Dr. Heidegger's study is like—in other words, it's unclear whether or not the narrator has actually ever been there.
Similarly, the narrator claims that there are “many wonderful stories” about the mirror in Dr. Heidegger’s study, but the narrator never actually verifies those stories to the reader. Elsewhere, the narrator attributes elements of the story to various parties, acknowledging that this isn't a firsthand account of the events described, using such phrases as “tradition tells us" and “according to some authorities.” The tale, then, has been patched together (as the narrator admits) from several different sources, some of which are merely speculative, hypothetical, or even just gossip.
The reader’s certainty in the story is further undermined when the narrator gives a glimpse of the scene from the perspective of a mirror, which shows three elderly men squabbling for the attention of an elderly woman rather than four young people. Given that everything in the story up until this point has suggested that Dr. Heidegger's friends actually turn into young people after drinking water from the Fountain of Youth, this detail forces readers to reexamine everything they've taken at face value in the world of the story.