The claim that "only my pain is real pain" confuses grammatical and physical impossibility. The person who makes this statement has not made a metaphysical discovery, he is just challenging our notation (notation refers to the way we use words). Wittgenstein says a similar confusion exists in the debate over whether we can classify "unconscious thoughts" as thoughts. This debate is over how we ought to use the word "thought," not over facts.
Discomfort with a certain notation and a desire to clarify a distinction often lead us to philosophical theorizing. In saying, "only my experiences are real," I am recommending a new notation. According to this new notation, the phrase "Brad has a real toothache" is nonsense, because no pain other than my own exists. The solipsist's claim that "when anything is seen, it is always I who see it" is equivalent to saying, "I am the vessel of life and only my experiences are real." It is logically impossible for anyone else to understand this statement: if someone else did understand it, that would mean someone else had the real experience of understanding, which is impossible according to the solipsist's logic.
Confusion stems from using "I" as both object and subject. For instance, the sentence "I grew an inch" uses "I" as an object. In the sentence, I recognize myself as a body in the world. The sentence "I have a toothache" uses "I" as a subject. In it, I recognize myself as the seat of experience. Wittgenstein suggests that using "I" as subject, when "I" does not refer to a physical person, leads us to think of the ego or mind as something distinctive that inhabits the body.
The discussion about what is really seen leads us to theorize about the existence of sense data. There is nothing wrong with talking about sense data. The problem occurs because the solipsist's insistence that "only this is really seen" does not oppose what is seen with what is not seen. The word "seen" becomes useless because it does not distinguish something from something else. Because we mean something other than our bodies when we say "I," we come to think an occult connection exists between meaning and our minds that contains all the possible uses of a word. We must recognize that meaning is something outside of the mind that is fixed by use.
Both here and in the Tractatus, Wittgenstein sees solipsism as a clear expression of a typical kind of philosophical confusion. Solipsism begins with the observation that my only access to the world is through my own personal experience, and recognizes that there is no way of knowing for certain that anything outside my personal experience even exists. Solipsism then concludes that reality is nothing more than my own personal experience, and nobody exists except me. Nobody is wholly solipsistic, for it would be impossible to live in the belief that everyone around you exists only as far as you perceive them to exist. However, the idea has fascinated philosophers because it is so difficult to disprove. We have no evidence to prove that anyone other than ourselves is real.
Wittgenstein's answer to solipsism in the Blue Book is similar to the answer he gives in the Tractatus. He does not try to disprove solipsism, or say that the solipsist is wrong, so much as he shows that what the solipsist wants to say cannot be expressed intelligibly in language. There are a number of ways we can use the word "see": we can talk about what I see, what you see, what I can't see, what can't be seen, and so on. If someone were to claim, "only what I see is really seen," this would not amount to a new discovery regarding the nature of vision, it would amount to an attempt to redefine the grammar of the word "see." Wittgenstein says this claim of the solipsists is simply a matter of recommending a new notation. That is, the solipsist wants us to use words differently than we do. Underlying Wittgenstein's discussion is an assumption of the public nature of language. Language can only be used to communicate ideas to others about features of our common experience. Any attempt to make blanket statements or to say something about the privileged nature of our own private experiences is bound to misfire.
Wittgenstein's discussion of notation, as it is used here, is unique to the Blue Book. He uses the idea of notation to suggest that the current grammar and usage of words is not fixed or definite, but a matter of convention. We can imagine other conventions, such as one according to which the phrase "really see" is used only in reference to what the speaker see. These conceivable other conventions are effectively different notations, different ways of using words.
Wittgenstein's discussion of notation bears the imprint of the Vienna School of logical positivists, with whom he had been involved a few years earlier. Logical positivism makes a distinction between the synthetic and the analytic. They says that synthetic statements talk about facts in the world and analytic statements define the rules according to which we will speak. The idea is that we have to agree on certain "framework principles"—analytic rules of language—before we can fruitfully discuss anything. This idea of "framework principles" probably influences Wittgenstein's discussion of notation: there is no right or a wrong notation, but it is important to agree upon a particular notation before trying to make headway. That is why Wittgenstein suggests there is nothing wrong with a notation according to which "really see" refers to what the speaker sees, so long as everyone agrees on this notation. The only drawbacks of this notation are that it is odd and arbitrary, and that it is liable to mislead individuals into thinking that they have some privileged position in the world.
It would be a mistake to call Wittgenstein a realist. He does not counter solipsism with the claim that solipsism is wrong, or say that there are other people and things that exist outside of our personal experiences. Rather, he suggests that the very debate between realism and solipsism is misguided and based on attempts to say things that cannot be said intelligibly. He does not argue that things outside of personal experience do or do not exist, he argues that we are misusing language when we think there is something to argue about regarding personal experience.