"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.