When you watch fish swimming around in very cold water, they look fine. Human beings have a protein, TRPM8, that reacts to cold and affects our nervous system, causing discomfort or even pain when the temperature goes down. But fish do not have any TRPM8 (Yong p. 138). Thus we can infer that fish do not sense cold in the way we do.
This does not mean that we know what cold is really like, while fish do not. Nor does it mean that our pain is nothing real, as if we can make it go away by disbelieving it. Nor does it mean that we know what it feels like to be a fish. But we can perceive a difference between species.
Long before anyone knew about proteins, the behavioral difference between us and fish was obvious enough that it served as an example for several thinkers who asked whether experiences like pleasure and suffering are subjective. More deeply, they asked what happiness is.
Japanese Zen Buddhism uses the term kyogai. Often translated as “consciousness,” it literally means “boundary” or “bounded place,” deriving originally from the Sanskrit word visayah, in the sense of a pasture that has a boundary. The Buddhist Abbot Mumon Yamada (1900-1988) taught:
This thing called kyogai is an individual thing. …. Only another fish can understand the kyogai of a fish. In this cold weather, perhaps you are feeling sorry for the fish, poor thing, for it has to live in the freezing water. But don’t make the mistake of thinking it would be better off if you put it in warm water; that would kill it. You are a human and there is no way you can understand the kyogai of a fish.
I think the upshot here is humility: if things seem and feel very different to creatures that have different senses, we cannot really know how things are. We should be compassionate, but that is harder than it may at first appear because it requires knowing what another feels. It would not be compassionate to move carp to a warmer pond. Our humility must temper even our compassion.
Aristotle wants to distinguish wisdom, which is knowledge of objective truths, from practical wisdom or phronesis, which allows us to act well. For example, “straight” (using the term from geometry) always means the same thing. The line that takes the shortest distance between two points is straight, regardless of whether any creature sees it as such–or sees it at all. In fact, a line would be straight even if there were no sentient creatures. Hence geometry is a part of wisdom.
However, says Aristotle, different things are healthy and good for people and for fish, and human phronesis involves doing the healthy thing for us, not for them. The “lower animals” also have practical wisdom because they also know what to do. If we try to convince ourselves that our phronesis is wisdom because we are higher than fish, we are foolish because there are things far more divine than we are (NE 1143a).
The upshot, for Aristotle, is that each creature has its own nature, and the proper definition of happiness is acting according to that nature. This means that a fish is happy if it swims around in the cold, not because that behavior feels good to it, but because happiness is accordance with nature. One distinguishing feature of human beings is that we can also know wisdom, or glimpses of it, by studying things higher than ourselves. Thus, for Aristotle, observing the behavior of fish does not really encourage humility. It directs us to identify our proper nature and its place in the cosmos as a whole.
Now here is a passage from Zhuangzi:
Zhuangzi and Huìzi wandered along the bridge over the Hao river. Zhuangzi said, ‘The minnows swim about so freely and easily. This is the happiness of fish’.
Huìzi said, ‘You’re not a fish. How do you know the happiness of fish?Zhuangzi said, ‘You’re not me. How do you know I don’t know the happiness of fish?’
Huìzi said, ‘I’m not you, so indeed I don’t know about you. You’re indeed not a fish, so that completes the case for your not knowing the happiness of fish’.
Zhuangzi said, ‘Let’s go back to where we started. When you said, “How do you know the happiness of fish”, you asked me about it already knowing that I knew it. I knew it over the Hao river’. (17/87–91)
I have virtually no knowledge of Taoism or its context, so it is risky for me to venture an interpretation. But I think the idea here is that neither of the men in the story can know the other, let alone the fish, and therefore all knowledge (including of one’s self) is illusory. However, Zhuangzi was right in the first place. “This” was the happiness of fish. He could not know its content or how happiness would feel to a fish, only that because fish were being fish, they were happy.
Ed Yong, An Immense World: How Animal Senses Reveal the Hidden Realms Around Us (Penguin Random House, 2022); Yamada as cited in Victor Sogen Hori, “Koan and Kensho in the Rinzai Zen Curriculum,” in The Koan: Texts and Contexts in Zen Buddhism (2000); Zhuangzi. The Complete Writings, translated by Chris Fraser (Oxford World’s Classics, p. 200). I translated Aristotle from the 1894 Clarendon edition on https://scaife.perseus.org/, but I have paraphrased here because the literal text is thorny. See also: some basics; Verdant mountains usually walk