When he needed to let off steam after a period of arduous work, Wittgenstein had two principal remedies: go see a movie at the local cinema—ideally a lowbrow American Western—or go for a long walk along the River Cam. On these rambles, he enjoyed the company of grazing horses—to whom he brought bread or sugar—and favored pupils, young men gentle enough in nature to calm his agitated mind, but bright enough to appreciate what was agitating him. Liberated from the stuffy atmosphere of the university, Wittgenstein’s mood was playful. On one outing with his friend Norman Malcolm, Wittgenstein paused at every tree they passed. With great solemnity, Malcolm reports, Wittgenstein “gave” him each tree “with the reservation that I was not to cut it down or do anything to it, or prevent the previous owners from doing anything to it: with those reservations it was henceforth mine.”

Wittgenstein once told Malcolm that a good philosophical work could be written that would consist entirely of jokes. Given Wittgenstein’s well-earned reputation for ferocious intensity, his appreciation of humor might come as a surprise. But many of his memoirists—he seems to have inspired almost everyone who met him to record their impressions—also recall his sense of humor. He could be impossibly demanding but his austerity was softened by a wry appreciation of the absurd.

That same humor surfaces occasionally in his writings. In one passage, Wittgenstein compares the idea of using memory to confirm something we’ve imagined to buying a second copy of the morning paper to confirm that what the first one said was true. In another, he envisions his right hand giving his left hand money, complete with a deed of gift written by the right hand and a receipt written by the left. In considering the social circumstances of pretense and lying, he asks why a dog can’t simulate pain: “Is he too honest?”

Wittgenstein doesn’t write this way because he’s a natural comedian. Rather, the kind of philosophical investigations he undertakes are structurally similar to a certain kind of joke. Wittgenstein constructs scenarios in which language is put to use in some activity or other—he famously calls these scenarios “language-games”—and interrogates the assumptions we bring to these scenarios, including the assumption that there’s any sense to be made of the scenario at all. Following these “games” often feels like following a joke in slow motion. In one much-discussed passage from his Philosophical Investigations, you find another version of the joke he shared with Malcolm. Wittgenstein imagines trying to note the recurrence of a certain sensation by writing an “S” in his diary on every day that he has the sensation. This scribbling turns out to be an empty ceremony, just like the gift of a tree. He’s made a mark in his diary all right, but he can’t do anything with this mark that we normally associate with the use of meaningful signs—he can’t, for instance, confirm two days later, when he’s inclined to mark another “S,” whether he’s using the sign correctly.

The similarity to his joke with Malcolm is striking: in both cases, Wittgenstein presents a form of words that seems to have sense but in circumstances in which it’s no longer clear what work those words are doing. Relieved of the responsibility of actually doing anything, the words spin free from the sense they initially seemed to make. Philosophical Investigations frequently plays on the tendency of philosophers to slip toward absurdity and nonsense. Perhaps it is the philosophical work that Wittgenstein alluded to as consisting entirely of jokes.

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Wittgenstein’s joke about the tree exhibits what he took to be wrong-headed about a certain approach to thinking about language. Wittgenstein thought that philosophers—his younger self included—had been misguided in considering language in the abstract, as a set of denotations and connotations that could be studied independently of the lives in which they were used. Instead, he proposed, we make sense of what words mean by understanding what we do with them. In “giving” the tree to his friend, Wittgenstein has gone through all the right motions of gift giving—at least so it would seem to a certain kind of philosopher. The joke is in the fact that giving a gift involves more than just the right words uttered in the right order—it also involves the speaker being in a position to give the property and its recipient being able to do things with their newly acquired goods. Likewise, it takes more than just money changing hands, and even a deed of gift and a receipt, to make an exchange of money a gift. Among other things, the hands have to belong to two different people.

Wittgenstein called his investigations “grammatical investigations.” But the grammar of language-games isn’t chiefly a matter of how words fit together. It’s about how our activities fit together so that those words have a use. The word “gift” makes sense alongside concepts of property, rights of use, exchange, and so on. The consequence of using a concept ungrammatically is simple: it means you’re speaking nonsense. And speaking nonsense can be funny. “Let’s ask ourselves: why do we feel a grammatical joke to be deep?” Wittgenstein writes. And adds: “And that is what the depth of philosophy is.”

Arguments can persuade you to change your position on something by showing why you’re wrong to make a certain claim, but Wittgenstein wants to show us that, when we slip the bounds of grammar, we’re not making any claims: the thought that we occupy a position at all is an illusion. What makes things even harder is that the territory of the grammatical isn’t marked by sharp boundary posts. Sense shades into nonsense in gradual and imperceptible ways. A lot of Wittgenstein’s philosophical work took place in these grammatical shadowlands, where our words start losing their grip. Things that seem to make sense don’t quite make sense.

Consider Wittgenstein’s honest dog. It’s easy to mistake his remark that a dog can’t simulate pain for an empirical remark, a claim about the psychology of dogs. That is, we might suppose that Wittgenstein is telling us about a scenario that we can readily imagine—a dog simulating pain—and then assuring us that, as it turns out, that scenario never comes to pass. But Wittgenstein wants to investigate the language-games in which the concept of dissimulation has a use until we realize that, given the way we use “dog” and “pain,” nothing in our imagination fits the description “a dog simulating pain.” The concept of dissimulation has a counterpart in the concept of honesty: someone who’s capable of pretense must also be capable of honesty. And someone who’s capable of pretense but never pretends is remarkably honest. If dogs could simulate pain but just don’t, that would be an argument for extolling the exceptional honesty of dogs. Wittgenstein’s question about the imagined dog—“is he too honest?”—uses the absurdity of that accolade to illuminate the less obvious absurdity of supposing that the dog’s inability to simulate pain is a matter of empirical fact.

The problem isn’t that Wittgenstein’s interlocutor is wrong to think that dogs can simulate pain. Rather, the interlocutor is wrong to think that there’s a thought there to be right or wrong about. Wittgenstein needs to jolt us out of the well-worn tracks our thought is inclined to follow and reorient us so that we can see our confusion more clearly. Jokes provide this jolt, prompting a shift not in belief but in perspective. Our train of thought gets derailed, but nobody gets hurt.

Wittgenstein didn’t think of philosophical nonsense as evidence of a derangement particular to philosophers. It’s rather evidence of a more general derangement in our thought and self-understanding that becomes particularly pronounced when we start doing philosophy. In effect, philosophers exhibit the same sorts of derangements as the protagonists of jokes, manifesting more patently the contorted self-understanding we are all subject to. Like the protagonists of jokes, philosophers earnestly go about business that we on the outside can see is absurd. But our laughter, if we’re honest, is a laughter of self-recognition.

We’re all fools, in other words, but fools who have become expert at concealing our folly, especially from ourselves. Clowns and caricatures do us the service of wearing on their sleeves the daftness we struggle to conceal. Wittgenstein wants us to see philosophers as providing a similar service. Imagining a conversation in a garden in which his interlocutor repeats over and over again, “I know that that’s a tree,” Wittgenstein imagines explaining the situation to someone who happens by: “This fellow isn’t insane. We are only doing philosophy.”