in advance.

One can understand why the old folk no longer want to be themselves. As well as being elderly, they're rather decrepit, and this is not a pleasant condition for anyone to find themselves in. Yet the way they conduct themselves when young again leads us to believe that Hawthorne is suggesting that it's not a good idea for us to wish to be someone else. We must make the best of who and what we are, no matter how old or infirm we may be. The guests' undignified behavior, though entirely predictable under the circumstances, is ultimately unacceptable.

The lesson that Hawthorne intends to show in his short story "Dr. Heidegger's Experiment" is that if people were ever to be given the opportunity to go back in time, they would also revert back to all the behaviors that characterized them at the time. In essence, people would likely not change. The key reason is simple: people are who they are, no matter in what place or time they exist. 

In the story, Dr. Heidegger intends to conduct an experiment where he will give four eccentric old people a youth elixir. He warns them, however, not to allow the feelings of youth to make them go back to committing the same foolish acts that were once dictated by their immaturity and naïveté.

In fact, he encourages the four participants to become beacons of respectability, maybe even inspiring other youths to avoid making bad choices and to start over again with better, respectable lives.  

...you should draw up a few general rules for your guidance, in passing a second time through the perils of youth. Think what a sin and shame it would be, if, with your peculiar advantages, you should not become patterns of virtue and wisdom to all the young people of the age!

Clearly, this is easier said than done. Once the participants feel young again, they immediately revert to the exact immature and self-indulgent desires that once marked their lives. They went back to the same issues, same arguments, and same weaknesses. 

They learned nothing. Rather than learning from their experiences as older people and applying them to their newfound youth, they instead indulged over and over in past bad choices. In fact, once the elixir wore off, they looked forward to finding the mythical fountain of youth, from which they planned to drink for days on end. 

Meanwhile, Dr. Heidegger may have learned that he was right all along: why try to go back in time if it means going back to the same issues that once haunted us? It is no surprise that, when the bottle of the elixir crashes and the liquid spills, Dr. Heidegger does not feel sorry in the least. 

Each of the old persons Heidegger choses for his experiment are known for some kind of immoral or illegal actions when they were young. Each one could be consider a symbol for a types of people known for scandalous behavior in the form of immoral sexual behavior, greed, political corruption or "lechery". Although Heidegger warns all of them that if they drink the water from the fountain of youth, they may revert to the same behavior that caused scandal when they were young. All of course say that they have learned from the past. However, when they do become young, they resort to their same old behaviors, which finally results in the destruction of the water which is supposedly giving them youth. Thus Hawthorne's lesson seems to be that people who engage in scandalous behavior generally do not learn from their mistakes, but continues to follow selfish
pleasure. This kind of pleasure is always short-lived, as witnessed not only by the behavior of the old people, but by Sylvia's rose and the butterfly. As Heidegger points out at the end, "if the fountain of youth flowed at my doorstep, I would not stoop to drink from it". Heidegger has learned to leave the past alone whereas the other still cling to it and the transitory pleasures it brought.

The moral of the story is to learn from one’s experiences so as not to repeat past mistakes. When Dr. Heidegger invites his friends, Mr. Medbourne, Colonel Killigrew, Mr. Gascoigne, and Widow Wycherly to his study to drink waters drawn from the “Fountain of Youth,” each of them reacts in an almost predictable manner, after drinking off the waters—Mr. Gascoigne gets all political, “ranting about patriotism, national glory, and the people’s right,” Colonel Killigrew gets to ogling Widow Wycherly’s full figure, Mr. Medbourne is all business and talks about “supplying the East Indies with ice” through some strange means, and Widow Wycherly is vanity itself, preening for hours before the mirror. One would expect that, given the chance to be youthful once more, each of Dr. Heidegger’s visitors would tread through life more carefully. It is almost like each one of them is asked the question: if you were to go back in time, would you do things differently? The four subjects have all accumulated a lot of experience about life, and one hopes that they have learned from their experiences. Mr. Medbourne lost all his wealth, in his prime, to a “frantic speculation” and lives like a beggar in his old age. Colonel Killigrew suffers from poor health because of a youth wasted on the “pursuit of sinful pleasures.” Mr. Gascoigne’s brand of politics has driven him into oblivion in his old age, and Widow Wicherly lives a “secluded” life, having involved herself in scandalous relationships with the wrong kind of men.

It is important to note Dr. Heidegger’s statement as he fills the glasses of his four friends: “For my own part, having had much trouble in growing old, I am in no hurry to grow young again. With your permission, therefore, I will merely watch the progress of the experiment.” The doctor knows that “growing young again” has its problems. This is why he carefully selects the subjects of his experiment, ensuring that each has a yearning for youthfulness. He would like to observe, however, if they are willing to redeem themselves from past mistakes, given the chance.

The moral of Nathaniel Hawthorne's tale, "Dr. Heidegger's Experiment," relates to the development of a person's moral character and feelings about old age.  In Dr. Heidegger's  experiment, in which he gives four friends a youth potion, he sees that, despite warnings and despite lessons about life, his four friends all reclaim their various character flaws along with their reclaimed youth.  In addition, he sees that the youth fades without having imparted anything of value to the friends' lives.  Heidegger also notes that their discontent with old age is amplified along with their heretofore partially dormant (due to the limiting restraints of old age) character flaws and they immediately concoct and pursue and impractical, wasteful scheme, that of finding the fountain of youth from which the water came.

Dr. Heidegger has a bust of Hippocrates in his lab and the narrator says that he would "consult" Hippocrates regarding difficult cases. Given the connection to the professionalism of Hippocrates and his "Hippocratic Oath," we might conclude that Heidegger is an upstanding physician who does the best he can for his patients. This is a moral code of professionalism.

But it is rumored that Heidegger also engages in mystical and magical experiments. These could be rumors and/or the creations of a "fiction monger" but they could be true as well. In either case, true or not, Heidegger uses the notion of magic in this particular experiment. Whether or not  that magic is real is irrelevant. The experiment, from his perspective, is not to see if these people become young again. It is to see how they will react and if they will learn anything from it. Since they have not learned from the mistakes of their younger days, Dr. Heidegger impresses upon them to learn something this second time around:

"Before you drink, my respectable old friends," said he, "it would be well that, with the experience of a lifetime to direct you, you should draw up a few general rules for your guidance, in passing a second time through the perils of youth. Think what a sin and shame it would be, if, with your peculiar advantages, you should not become patterns of virtue and wisdom to all the young people of the age!"

In the end, Dr. Heidegger learns to accept the passage of time. Of Sylvia's flower, he says he loves it now (withered) as much as when it was fresh. But his four subjects do not learn this lesson.

On one hand, we might conclude that Dr. Heidegger knew these people well and might have assumed they would remain vain and mentally immature. On the other hand, he might have actually been trying to teach them to accept old age and, with it, wisdom. If this is the case, then he would certainly be upholding the Hippocratic Oath because his intention would have been to help these people to learn from their past mistakes and to accept the wisdom of old age. If this is the case, in this particular experiment, his moral philosophy is based upon helping people overcome vices, regret, and depression.

A lesson to take away from this story may be the folly of youth. Before the experiment is conducted, Dr. Heidegger implores his subjects to, with their newly restored youth, act with the wisdom of someone who has already endured all of the years of life. His subjects laugh at the idea of even having to be told this—as if they would ever repeat the mistakes that made them so miserable.

However, as soon as their youth is restored, they begin once again to act foolishly, all of the men fighting over the Widow Wycherly. In their youthful ecstasy, they have completely forgotten the woes that will eventually be coming for them again, and they once again consider themselves to be immortal. Dr. Heidegger, who represents some wisdom in this story, would never drink from the fountain of youth. He states, "having had much trouble in growing old, I am in no hurry to grow young again."

Everyone can take different things from the story, but one possible lesson is the idea of being content or happy about one's situation in life, in this case particularly about the age.  The old men become young and then compete for the beauty of the (now) young woman in the group.  They are absolutely horrified when they grow old again and plan to seek the water for themselves because they cannot be happy with their current dilapidated state.

It was easy for the doctor to watch them and learn the lesson, but much more difficult for the subjects that actually felt the change and felt the vigor of youth and then desperately longed to continue the experience.

Dr. Heidegger's experiment is conducted with four willing participants: the Widow Wycherly, Colonel Killigrew, Mr. Medbourne, and Mr. Gascoigne.  All are of an advanced age and eager to try a substance he identifies as fluid from the Fountain of Youth. Because all four have made poor choices in their past and suffered negative consequences, Dr. Heidegger counsels them to use the wisdom gained from their life experience and "become patterns of virtue and wisdom to all the young people of the age!" Because Dr. Heidegger believes he understands human nature and expects that the four participants in his experiment will make the same mistakes, he offers this advice.

The four find his advice "ridiculous" and unnecessary; the implication is that anyone given the chance to relive their past would naturally sidestep their youthful mistakes and missteps to avoid trouble and regret.

So many of Hawthorne's tales are richly allegorical and have a definite didactic purpose. This tale is certainly no exception. The key to establishing the theme is therefore to identify what message or lesson is communicated through the action. Clearly, the focus is placed on age early on in the story through the reference to the somewhat shady and mysterious background of Dr. Heidegger and his lost love, whose death he is implicated in. Note how the rose is introduced as a symbol of his relationship and his love for his former lover:

"This rose," said Dr. Heidegger, with a sigh, "this same withered and crumbling flower, blossomed five and fifty years ago. It was given me by Sylvia Ward, whose portrait hangs yonder; and I meant to wear it in my bosom at our wedding. Five and fifty years it has been treasured between the leaves of this old volume."

Just as the rose has withered, so has Dr. Heidegger and his guests have withered. Yet, even though Dr. Heidegger manages to restore the beauty and vigour of the rose, in the same way that his guests have their youth restored, the transitory experience shows him the benefits of old age and the wisdom that comes with it instead of the follies that inevitably characterise youth. Note what Dr. Heidegger comments about the Water of Youth and its loss after seeing his guests repeat their youthful indiscretions:

Well--I bemoan it not; for if the fountain gushed at my very doorstep, I would not stoop to bathe my lips in it--no, though its delirium were for years instead of moments. Such is the lesson ye have taught me!

The subject of this story is therefore age and our perhaps natural desire to wish to go back through time to our youth and our naive belief that we would not repeat the same mistakes. This story questions such beliefs and also demonstrates the richness of old age, challenging our views about aging and growing old.

Dr. Heidegger has made a truly remarkable discovery. He's discovered nothing less than an elixir of youth. This happened when he came across a Fountain of Youth in southern Florida. Anyone who drinks the water from this fountain will become young again. The good doctor wants to try this elixir of youth on his friends, none of whom are getting any younger.

In the event, Heidegger's experiment works very well—a little too well, in fact. Once his friends have drunk the water from the fountain, they are gradually transformed into their younger selves, full of vim and vigor.

However, Dr. Heidegger is none too impressed by what he sees. Although the elixir of youth has made his friends feel a good deal more youthful, the doctor is somewhat appalled at their childish behavior. Far from being young, they're just old people acting as if they were young, and there's something rather undignified about that.

But the doctor's friends don't care; they love how the magic water makes them feel. Even though its effects wear off after a relatively short time, they still want to drink more of the stuff and resolve to go to the Fountain of Youth in Florida to get their hands on some more of it.