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.