A key theme in Hawthorne's short story is foolishness, and whether a fool can ever change. It is often said that age begets wisdom, and essentially Dr. Heidegger's experiment is to test this hypothesis. Are the older people in the story actually better versions of the people they were in their youth—the bombastic politician, the drunk, the gambler, and the flirt? Or is it simply that age has rendered it impossible for them to behave in this way? Heidegger seems to have an inkling that the fools, at heart, remain fools, and this is borne out by the story. The guests at the dinner party have not learned from their years of experience: as soon as they are enlivened once more by the Fountain of Youth, they engage in reckless behavior, while their friend, the doctor, simply observes the experiment. His final comment seems to indicate that this is what he expected, as he states he would not drink of the Fountain having witnessed this. Accordingly, the fools have learned nothing, and immediately want to set out for Florida in search of the fountain itself.

Is there really a Fountain of Youth at all? Hawthorne leaves it deliberately ambiguous: the guests are at first skeptical, and then become convinced, a journey on which the reader accompanies them. At first, it seems a mere trick, until the narrative shows the old Widow seeing her own face "bloom" again in the looking glass. But what does this "mirror" signify? Is the Widow seeing only what she wants to see, buoyed by her own foolishness and the power of suggestion? When the other mirror in the study shows the older people as simply withered elderly figures behaving like youths, the narrative drives us to question our own understanding of what is going on. Are the old people really young again, or are they—and we—fools to believe in this possibility?

Beauty is, as cliché as it is, a subjective concept. The characters in “Dr. Heidegger’s Experiment” seem to think that beauty is only found in youth. They behave without inhibition when they believe they are “beautiful” again. They fail to see that they are the same people with the same features, merely aged. Perhaps Hawthorne has pointed out a commentary on the shallow nature of people. Sometimes, people can only see what is directly in front of them (such as surface-level beauty or youth) and fail to dive deeper into what is meaningful. If the only things in life that are considered beautiful are physical, then beauty truly is fleeting. But, if we appreciate life in its many facets and forms, we might be surprised by the beauty that is found there. Interestingly, Dr. Heidegger seems to uphold this mentality. He comments on his beloved rose after it has withered, saying, "I love it as well thus as in its dewy freshness." He recognizes that there is beauty in the rose’s prime and in its memory. He has no unrealistic expectations of the rose: he knows it is half a century old. Even when it is withered, it is beautiful in his eyes because of what it represents.