"Dr. Heidegger's Experiment" is rooted in a rather pessimistic view of human nature. The story argues that people are, for the most part, fools. They don't learn from their mistakes, they're generally petty, and we can't expect anyone to change for the better. In this story, foolishness is particularly associated with youth, or at least a youthful state of mind. Hawthorne does provide a counter-example to his foolish characters in the form of Dr. Heidegger, but even this character has his sinister side.
In illustrating the foolishness of his characters, Hawthorne condemns his readers as fools as well.
The four guests are two-dimensional caricatures, not fully developed, three-dimensional characters.
"Dr. Heidegger's Experiment" explores questions of age and behavior. What does it mean to be old? What does it mean to be young? What is the difference between defining age physically, and defining it mentally or emotionally? One interpretation of the text suggests that age is a state of mind; if one believes one is young, one will act accordingly. The story's moralistic side argues that youth is associated with folly, but offers no hope for redemption in older age, either.
Dr. Heidegger already knows what the outcome of his experiment will be, because he already understands the nature of youth and folly. He only subjects his guests to the experiment for his own sadistic amusement.
The authorial tone of "Dr. Heidegger's Experiment" is a pessimistic one.
"Dr. Heidegger's Experiment" is a story of illusion, deception, and doubt. The title character makes use of theatricality, wishful thinking, and even alcohol (in one interpretation of the story) to make experimental subjects of his friends. This illusory trickery even seeps into the narration – the narrator's shadowy evasions raise similar questions for the reader of what is real, what is fictional, and, most interestingly, whether or not it matters.
"Dr. Heidegger's Experiment" is intentionally ambiguous as to whether or not the water in the vase really is the elixir of life. The question is ultimately beside the story's main point.
The unreliability of the narrator serves to place the reader in the same state of uncertainty as Dr. Heidegger's guests.
"Dr. Heidegger's Experiment" is the story of four elderly friends who are transformed – or at least think they are transformed – back to young, vivacious individuals. The text plays with the idea of what it means to be transformed physically as opposed to mentally, and which actually takes place in the story is subject to debate. Hawthorne also asks whether we can learn from a transformation, particularly one as ephemeral as that which takes place in this story. Pessimistically, the narrative seems to conclude that we can not.
The setting sun is an important key to understanding the "transformations" taking place inside Dr. Heidegger's study.
The guests' transformation proves that age is a state of mind, not a physical state.
Whether or not the eerie elements of "Dr. Heidegger's Experiment" are actually supernatural is subject to debate. It's a question of theatrical showmanship vs. genuine superstitious belief. The theme creates a mood of doubt and forces the reader to ask some difficult questions about the nature of reality – at least as defined within the narrative. It also severely complicates our understanding of the title character, who has at least one foot in a sinister, supernatural realm.
Hawthorne condemns Dr. Heidegger for playing God.
Dr. Heidegger's character is both venerable and Godly. Hawthorne paints him as an admirable, if other-worldly, figure.