In the following examples, unless otherwise stated, the computers are traditional ones without cameras.

Before we dive into a series of brain-teasing philosophical questions, let’s begin with a gentle warm-up problem. You are to play the role of Cat A. Your companion, Cat B, has disappeared while playing hide-and-seek, and there is a suspicious opaque box in the room. What can you do, given that you are not allowed to open the box? Let's first assume that the box is thin enough that you can hear any sounds coming from inside. In that case, you can try calling out and see if there is any response. If the box makes a "meow" sound, we can infer that Cat B is inside the box. However, since we are playing hide-and-seek, it is likely that Cat B will not respond. Therefore, we can walk around and try to push or hit the box. If you hear the sound of paws scratching, a tail swishing, or more "meowing", we can reasonably conclude that Cat B is inside the opaque box.

Let’s summarize the approach: when faced with a box and trying to determine whether Cat B is inside, we can probe the box from the outside and observe the reactions to different stimuli. By testing these external responses, we can infer the presence of Cat B’s soul inside the box.

Now, remember this method you’ve used, as we move on to the more complex philosophical question below.

The Soul of a Cat Entered the Body of a Dog

Let’s start with a thought experiment. I, Doctor Strange, bring out a dog and claim that its soul has been replaced with that of a cat. You surely wouldn't believe me, but as Doctor Strange, you can’t dismiss it entirely. What should we do? Clearly, we could check whether it barks or meows, but that’s too simple. Let’s assume the dog’s vocal cords can’t make a meowing sound. What else can you do?

In this case, you can conduct experiments on this dog. You might throw it a fish and observe its reaction. Then try throwing it a bone, a mouse, excrement, a cat toy, a ball of yarn, a laser pointer, a crocodile, or catnip… Keep experimenting with all possibilities, and if its reactions align with those of a cat, you can conclude that the soul of a cat resides in the body of this dog. And, fortunately, I have a book that explains how to interpret a cat’s various vocalizations…

I am now able to understand whether a cat is hungry, angry, happy, etc., by its different meows... [8]

The more tests you run, the more reliable your conclusion becomes. If you exhaust all the possible experiments and find no contradictions, you can be certain that this animal is the cat’s soul, regardless of whether it’s a dog or not. Perhaps I’m not Doctor Strange, and it’s actually just a dog that I trained to behave like a cat, but that would be an undecidable problem, so we still treat the soul as equivalent to that of a cat. This concept is essentially what the Yoneda Lemma [7] describes. I’ve rewritten the dog’s expressions to match those of a cat, so I’ve essentially replaced its soul with that of a cat’s, whether by training or by magical means.

The Soul of a Suona Entered the Body of a Flute

Don’t be fooled by the fact that the previous problem requires exhausting all possibilities to solve. In certain specific cases, the problem can become surprisingly simple. Now, Doctor Strange presents you with a new puzzle: What if the soul of a suona (a traditional Chinese wind instrument) enters the body of a flute? Before answering this question, let’s first consider what it really means. The soul of a cat is easy to understand, but what does it mean for the soul of a suona? How is it different from the soul of a flute? Both the suona and the flute are wind instruments, and while their playing techniques differ, the soul belongs to the player, not the instrument itself. After thinking it over, we can only conduct an experiment: blow a breath into it and see whether the sound produced is that of a suona or a flute. However, unlike the earlier case of the cat’s soul, determining the suona’s soul can be settled with just one experiment. This is because, for time-invariant linear systems (which most musical instruments are), the impulse response contains all of its "soul." In mathematical terms, this is called the Green's function. The breath you blow is the impulse, and the sound produced is its response. Perhaps in reality, it’s a high-quality digital sound system simulating this impulse response, but then it becomes an indeterminate problem. From this perspective, we can also say that a sound system can possess the "souls" of all instruments (excluding the player’s soul), because it can simulate all of their impulse responses — and it certainly seems to be the case.

The Soul of a Human Entered the Body of a Computer

Although I’ve introduced the setup where Doctor Strange uses magic in the examples above, in reality, no magic is needed. I’ve personally experienced the feeling of a soul being trapped. Once, I was speaking in a Twitter Space (without a typing function), and due to a disconnection, my microphone couldn’t transmit my speech. However, I wasn’t completely offline because I could still hear others speak. Since Twitter has a clapping function, I frantically clicked the clapping button to indicate that I was still online. Obviously, the solution to this problem would be to reconnect. But what if we kept it this way? Isn’t it like my soul is trapped in the computer? If both parties cooperate, I could cleverly use the rhythm of clapping to tap out Morse code to express myself. As long as I have the ability to clap, I could communicate with the outside world and prove that I have a human soul. The outside world could detect my human soul through various forms of communication. As for whether it’s really me behind the computer or just an AI, that depends on whether the person on the other end can find any clues. If they can’t exhaustively find anything, then it becomes an undecidable problem. According to the Yoneda Lemma, regardless of whether it’s truly me or not, it’s equivalent to my soul.

It’s just this small clapping function, but if you remove it, you lose all ways to prove that my soul exists. According to the Yoneda Lemma, my soul disappears. In fact, brain death is similar: your life signs are still present, but you’ve lost all means of expressing yourself (including body language and facial expressions). According to the Yoneda Lemma, your soul also disappears — becoming a soulless being.

Do Plants Have Souls?

Although brain death and being in a vegetative state are different, since we’re discussing souls, let’s also consider whether plants have souls. As mentioned earlier, while animals don’t have symbolic language like humans, they can still express their souls through sounds and body language. So, do plants have their own language? Aside from a few plants, like the sensitive plant (Mimosa pudica), which can respond to stimuli in a way humans can perceive, most plants do not have a language that humans can directly understand. From this perspective, plants do not possess an animal-like soul that is perceptible to humans. To explore whether plants have a soul that humans cannot perceive, we would need more advanced measuring instruments. Perhaps plants communicate with each other in their own language, who knows? (And please don’t bring up that experiment where plants wilt when cursed at.)

However, at the very least, we can confirm that plants have a biological soul: I can burn them with boiling water, and they respond with death, which is consistent with the feedback of living organisms.

After all, I don’t have a plant’s soul, so strictly speaking, I’m not qualified to judge a plant’s soul. However, since I have a human soul, an animal soul, and a biological soul, I am capable of judging the soul of a human, a cat, or from a biological perspective, the soul of a plant.