This book is a story about philosophy, parallel universes, existence, and living—but above all, it is a story about regret.

Regret is an age-old cliché. When it comes to this emotion, we often hear people declare: “I never regret the past, because regret serves absolutely no purpose in the present.” Everyone aspires to live a life devoid of regrets, but is that truly achievable in the real world? To explore this question, we must first define what regret actually is.

The human brain possesses the unique ability to recall the past and envision the future, allowing our minds to traverse unrestricted through time—backward and forward among the past, present, and future. We can even mentally backtrack to a specific event in the past, anchor ourselves at that exact moment, and imagine how the future might have unfolded from there. In other words, our imagination allows us to mentally reconstruct past events. Consequently, branching out from that singular moment in the past, two distinct selves emerge in two different worlds: a virtual self inhabiting the reconstructed world, and a real self living in the actual world. Naturally, we compare these two versions of ourselves. When we perceive that the self in the real world is worse off, we experience pain and find ourselves blaming the choice we made at that past juncture. This entire cognitive and emotional architecture is what we define as regret.

Once we comprehend the true nature of how regret is generated, it becomes glaringly obvious that the experience of regret is universal. Furthermore, it is utterly impossible to live a life entirely free from it.

Why is regret unavoidable? Because in the exact moment of the past, we had no way of knowing that we would come to rue the choices we were making. Of course, some might counter: “There were times when I acutely felt that I might regret what I was doing, yet weighed down by the emotional gravity of the moment, I felt compelled to proceed anyway.” Indeed, we cannot deny the existence of such scenarios. However, looking back, if you had already predicted future regret at that past moment, does that not precisely prove the sheer inevitability of regret itself?

Since regret is an inescapable process, and since it inherently brings feelings of pain and guilt, how then should we coexist with it?

First, face it with equanimity. A regret-free life simply does not exist; every single person carries their own catalog of regrets. Regret is not a matter of shame, so please look your regrets dead in the eye. Second, strive to let go. If there is no such thing as a perfect life, then by the exact same token, there is no such thing as a worst-case life either. Even if one were to argue that our current life is merely the karmic retribution of a series of “wrong” choices, at the very least, every beautiful thing we possess right now is also the continuation and reward of that very same past. Finally, refine the future. The duration and depth of the negative emotions brought by regret depend entirely on whether we take positive, proactive action. By reflecting on our regrets, we can make better decisions in our future journeys. If we do nothing, regret will vividly and repeatedly loop in our minds without end—trapping us, forever wandering, within the corridors of the Midnight Library.