Why you can never truly know what someone else feels
Ludwig Wittgenstein asked us to imagine a group of people, each holding a box. Inside each box is something they call a "beetle." No one can look inside anyone else's box; they only know what a beetle is by looking at their own. Wittgenstein argued that even if everyone’s box contained something completely different, or was entirely empty, the word "beetle" would still function in their language. It illustrates how our private experiences cannot be directly shared.
The Illusion of an Entirely Private Language
Most people assume that words like pain, itch, or joy point directly inward to private experiences that no one else can ever witness. Under this common view, each person learns what pain is by feeling their own physical discomfort, assigns a mental label to that inner occurrence, and then uses the spoken word to describe it to others. Because another person cannot feel your precise sensation, it seems to follow that language serves as an imperfect bridge between sealed, inaccessible inner worlds. This philosophical perspective, heavily influenced by René Descartes, treats the mind as an inner theater to which only the owner has direct access, leaving the existence and nature of other minds open to perpetual skepticism.
In his posthumously published 1953 work, Philosophical Investigations, Austrian-British philosopher Ludwig Wittgenstein directly challenged this view through what is now known as the private language argument. Wittgenstein did not merely suggest that communicating inner feelings is difficult; he argued that a purely private language—one whose words refer exclusively to what can only be known by the speaker and cannot be understood by anyone else—is conceptually impossible. If language is to mean anything at all, its rules must be public, verifiable, and rooted in shared human practices.
The Beetle in the Box
To demonstrate why private sensations cannot serve as the foundation for the meanings of words, Wittgenstein introduced a famous thought experiment involving a beetle in a box. Imagine a community where every individual possesses a small box. Inside each box is something that the owner refers to as a beetle. The crucial condition of this scenario is that no one can ever look inside anyone else's box. Each person determines what the word beetle means solely by looking at the contents of their own container.
Wittgenstein pointed out the strange logical consequence of this arrangement. It is entirely possible that everyone has something radically different inside their box. One person might have a pebble, another an insect, and another a piece of paper. The contents might even change continuously without anyone else knowing, or some boxes might be completely empty. Yet, despite these vast, undetectable differences, the word beetle would still function perfectly well in their everyday conversations. People could trade boxes, talk about their beetles, and navigate social life without ever verifying the actual contents of any box other than their own.
From this, Wittgenstein drew a striking conclusion: if the word beetle has a meaningful use in this community, the actual object inside the box has nothing to do with that meaning. The private object cancels out, or drops out of consideration as irrelevant to the language-game. If sensation words operated purely by naming private, uncheckable inner states, those inner states would be just like the beetle—incapable of establishing or anchoring the shared meaning of our words.
The Man with the Diary and the Problem of Rules
To show why an individual cannot even invent a private language for their own personal use, Wittgenstein offered another scenario: a person who decides to keep a diary of a recurring sensation. Every time this particular sensation arises, the diarist writes down the letter S in a notebook. The diarist attempts to establish a private definition by concentrating their attention on the feeling, mentally pointing to it, and deciding that S will henceforth name that precise sensation.
Wittgenstein argued that this ceremony of private ostensive definition cannot actually establish a rule. To use a word or symbol correctly, there must be a genuine distinction between correctly following a rule and merely thinking you are following it. In the case of the solitary diarist, there is no independent standard or external criterion to verify whether the feeling experienced on Tuesday is genuinely the same sensation that was labeled S on Monday. Memory alone cannot serve as an independent check on memory; that would be like buying several copies of the morning newspaper to confirm whether the story in the first copy was true.
Without an external criterion of correctness, whatever seems right to the diarist will be treated as right. But when whatever seems right is automatically accepted as right, the very concept of being right loses all meaning. A rule that cannot be broken or checked by any standard outside one's own immediate impression is not a rule at all. Consequently, the letter S fails to acquire any genuine meaning, even for the person writing it.
How Sensation Words Actually Work
If sensation words do not gain their meaning by naming hidden, private objects, how do we learn and use them? Wittgenstein argued that language is inextricably tied to outward human behavior and social interaction. When a young child is hurt, they do not invent a private mental label for their state. Instead, they produce natural, unlearned expressions of pain, such as crying, wincing, or holding the injured limb. Adults intervene and teach the child new behaviors, including verbal expressions such as exclaiming that it hurts.
In this framework, the verbal expression of pain does not describe an invisible inner object; rather, it replaces and refines the primitive, natural expression of discomfort. Words for sensations are learned through public criteria—observable behaviors and the circumstances in which those behaviors occur. The community can evaluate whether someone is using the word correctly because the word is tied to visible criteria, such as a bleeding scrape, a groan, or a sudden flinch.
This shared network of language and action is what Wittgenstein referred to as a language-game, which is embedded within a broader form of life. Sensation language functions reliably not because we can peek into the private chambers of each other's minds, but because our words are woven into shared human activities, mutual recognition, and observable reactions.
Dismantling Solipsism and Cartesian Doubt
Wittgenstein’s argument fundamentally upends the philosophical position known as solipsism—the belief that one can only be sure of the existence of one's own mind. Traditional Cartesian skepticism starts by assuming that an isolated thinker can possess a fully formed, coherent language to describe their private thoughts, sensations, and doubts, while remaining entirely uncertain about whether an external world or other people exist.
The private language argument reveals that this skeptical starting point is logically flawed. A solitary thinker could not possess the concepts of doubt, sensation, or thought in the first place without relying on rules that are inherently social and public. To doubt whether other minds exist requires using a language whose very existence depends on public criteria and an external community. Skepticism about other minds relies on the very shared linguistic framework it attempts to undermine.
Debates and Differing Interpretations
Wittgenstein’s arguments sparked intense debate within twentieth-century philosophy, leading to divergent interpretations among subsequent thinkers. One major debate concerns whether Wittgenstein was advocating a form of behaviorism—the view that mental states are nothing more than physical behaviors. While some critics accused him of denying the reality of subjective experience, defenders argue that he was not denying that people feel real, agonizing pain. Rather, he was denying that the grammar of our sensation words works on the model of an object and its designation.
Another influential interpretation was proposed by philosopher Saul Kripke in his 1982 book Wittgenstein on Rules and Private Language. Kripke argued that the core of the private language argument is a general skeptical paradox about rule-following that applies to all language, not just sensation words. According to Kripke's reading, no fact about an individual's past mental history determines what they mean by any word, leading to a community-based solution for linguistic meaning. While scholars continue to debate whether Kripke's reading accurately captures Wittgenstein's original intent, the private language argument remains one of the most enduring and widely discussed challenges to traditional theories of mind and meaning.
Key takeaways
•Wittgenstein argued that a purely private language—referring to inner experiences that only the speaker can know—is impossible because meaning requires public standards of correctness.
•The 'beetle in a box' thought experiment shows that if sensation words referred only to uncheckable inner states, the actual inner state would be irrelevant to the public functioning of the word.
•A solitary individual cannot establish a private linguistic rule because without external criteria, they cannot distinguish between correctly remembering a rule and merely thinking they remember it.
•Sensation words acquire shared meaning not by privately labeling internal objects, but by replacing and expanding natural, observable human expressions within a community.