Wittgenstein now turns to the topic of personal experience, explaining that he has kept away from it until now for fear of introducing further difficulties that would cast in doubt the results of the discussion so far. Philosophical work is like stacking books that lie in a mess on the floor: it is worth grouping a series of books together on a shelf even if later we will have to move them to a different shelf. It is important to achieve an initial stage of organization even if later we will have to displace it.
A common philosophical temptation is to equate personal experience with reality and conclude that we can only talk about the immediate objects of our own personal experience. Someone who does this is called a solipsist. This conclusion leads us to believe that objects are really fleeting and vague components of sensory experience, and that language deceives us by suggesting that objects are solid and stable and exist outside our own perception.
The discovery that solid objects are made up of atoms with vast spaces between them leads us to conclude that a wood table is not really solid, as we once believed. This conclusion renders the word "solid" useless, as we can no longer use it to distinguish a normal wood table from a table of rotting wood, or from a heap of sand. The physicist's discovery does not teach us that wood tables are not solid, it gives us a physical definition of solidity. Philosophy often makes the mistake of using words in a way that empties them of meaning. If, in reference to the idea that reality consists of personal experience, we assert that all of reality is vague, we have then emptied the word "vague" of meaning, because we have made it impossible to talk of anything as being "not vague."
When I claim that only my personal experience is real, I draw a line between the world of material objects and the mental world of personal experience. The solipsistic claim that only my personal experience is real raises the question of how I can believe that other people experience pain. A realist, who replies that we can easily imagine what someone else's having pain would be like, does not answer the solipsist's question, but merely bypasses it by using "imagine" and "have" in unusual ways. To imagine A has a toothache is entirely different than imagining A has a gold tooth. I can form a mental image of a gold tooth and then compare that image with reality. There is no equivalent way that I can locate a toothache in someone's mouth or form a mental image of that toothache. At best, I can imagine how A might behave if he had a toothache, which is an indirect representation of the pain.
We misuse the word "know" when we say, "you cannot know another person's pain, you can only conjecture it." The word "know" is useless here, because there is no sense of what "knowing another person's pain" would be like. We must take care not to confuse physical and grammatical impossibilities. Words like "can" and "must" often accompany grammatical rules, such as "the colors blue and green cannot occupy the same place simultaneously." We feel we are only stating the physical impossibility that "they are in the same place" is ever true, but in fact we are stating the grammatical impossibility that it makes sense to speak of two colors existing in the same place.
There are three levels on which we can read the distinction between grammatical and physical possibility. On the first level, it is more definite to rule something out based on grammar rules than to rule it out based on experience. If I say, "I cannot see bacteria," I am stating that my visual apparatus is not suited to seeing such small objects. If I say, "I cannot see your toothache," I am stating a grammatical impossibility of seeing toothaches. On the second level, we tend to confuse grammatical and physical impossibility. They share a common grammatical form ("we cannot" or "it is impossible"), so one kind of impossibility is easily confused with the other. Wittgenstein gives the example of the statement "the colors blue and green cannot occupy the same place simultaneously." When we hear this statement, we see a mental image of a physical barrier separating the two colors. We visualize this impossibility in physical terms rather than recognize it as a statement of grammar.
On the third level, grammatical impossibility is of a totally different kind than physical impossibility. They are not simply analogous forms; grammatical impossibility is not simply a sterner form of physical impossibility. While physical impossibility states a limitation of sorts, grammatical impossibility states a rule of our language. The phrase "I cannot see bacteria" tells us something about what the world is like. The phrase "I cannot see your toothache" teaches us how we use the word "toothache." If Jane holds Dick's mouth open, shines a light into it, probes about, and then says, "I cannot see your toothache," she is stating that seeing his toothache is physically impossible. By using the phrasing she does, Jane says she was looking for something but she could not find it. This scenario is logically absurd, because there is nothing to look for and nothing to find. Toothaches are not visible. If someone learning English has an inflamed gum and asks, "can you see my toothache?", I might reply, "I can see a gum inflammation; I cannot see your toothache." Here, in saying, "I cannot see your toothache," I am explaining how we use the word "toothache" and saying that we cannot see toothaches.
If I say, "I cannot know your pain" as a grammatical rule, it makes sense. If we misread this as a statement of physical impossibility, however, then we imagine there must be such a thing as "knowledge of your pain" to which I do not have access. From this I might erroneously infer that I have made a philosophical discovery that I have knowledge only of my own pain, and that all my knowledge is confined to my own personal experience. Solipsism results.
One significant difference between grammatical impossibility and physical impossibility is that the negation of a physical impossibility is conceivable. We can imagine what it would be like to see bacteria, but we cannot imagine what it would be like to see a toothache. Except when stating grammatical rules, the negation of a statement must be conceivable, for only then will words make sense. For instance, the word "solid" has no sense if we argue that nothing is solid. The word is only useful if it helps us to distinguish certain kinds of objects from certain other kinds of objects.
This observation highlights a significant theme that runs through every part of Wittgenstein's philosophy. Roughly put, there is no external perspective we can take with regard to the world. We exist only in the world, and can use language only to distinguish certain things in the world from certain other things in the world. For instance, it makes no sense to say, "everything is in flux," because the word "flux" only makes sense in contrast to its opposite, "stable." If everything is in flux, I can no longer use the word "flux" to distinguish certain things or processes in the world from others that I take to be more stable. The word loses its meaning when we try to make a blanket statement about all of reality.