As a metaphor for the world we live in, consider a city with very strict privacy laws. In this city, everyone is entitled to privacy in his own home, and is forbidden to surrender that privacy by admitting anyone else to his home, or sell his home and move to another. The streets are public: there people meet and mingle and converse. The houses are private, each accessible only to one. These arrangements would affect the structure of knowledge in this city.
Things in the streets, like fire hydrants and stoplights, would be visible to all. When they were discussed, people could readily allude to specific examples. If someone suspected that the reports he heard about a fire hydrant or a stoplight was untruthful or erroneous, he could go and check for himself. Curious, clever people who discovered new things about fire hydrants and stoplights, or discovered facts that overturned eventual wisdom, could demonstrate their discoveries to others. Everyone who undertook to study them could readily benefit from the accumulated knowledge of others, satisfying himself of its truth by observing the demonstrations. Establishing terminology and consensus about lawlike generalizations would be straightforward.
Some things in the houses might be common to all of them, e.g., kitchen sinks and showers. Still, it would be difficult to establish any general rules about these. If Person A observes that his kitchen sink has properties x, y, and z, he cannot be sure, merely on the basis of experience, whether those are general features of kitchen sinks and showers, or peculiar characteristics of his own. He can only investigate this by asking other people about their kitchen sinks. If other people tell him, "Yes, my sink has property x, too," he may conclude that he has discovered a general feature of sinks, whereas if they tell him, "No, mine is not like that," he may decide that property x is a distinctive feature of his own sink. Yet he may still be suspicious. On the one hand, if people confirm his claim, he may suspect that they are merely suggestible or polite, and that if he could observe their sinks firsthand he would see something different. On the other hand, if people deny his claim, he may think he has not explained it well, or his interlocutors are obtuse, and that actually their sinks do have the same feature.
It will be especially frustrating for a man who, having great curiosity about kitchen sinks and showers, studies them in great detail and becomes an expert plumber. When he expounds his elaborate explications to his less curious and ingenious neighbors, to see if his findings match their experience, they may shrug and say, "I don't know." He may be fortunate and find other autodidact plumbers to converse with. But, even if he and his fellow enthusiasts are able to converse productively, he may suspect that his sample is biased, and that it is something strange about the plumbing in these houses that sparked their unusual shared interest.
One barrier to discussing the houses in our city is linguistic. No one can ever establish the meaning of a term that describes something in the house by saying: "See that? That's a sink." Instead, they will have to talk about the things in the houses by analogy to things in the streets. But this is likely to give the discussion a weirdly censored character. We may suppose that there are drinking fountains in the streets, so conversations about interior design tend to rely on the street experience of drinking fountains to explain the house experience of sinks. Indeed, very likely, sinks will simply be referred to as (interior) drinking fountains, and if drinking fountains are rare in the streets, the word may almost always be used to refer to kitchen sinks, yet everyone will maintain an awareness of the metaphorical origins of the word. There may, however, be other features of the interiors where external metaphors are much less satisfactory-- our city-dwellers might find they can only refer to ceiling lamps as (interior) suns, and to carpets as lawns-- and still others where even passable external metaphors are elusive, e.g., wall sockets.
Of course, there will be some interior features that differ between houses. Jacuzzis, sunrooms, skylights, and wine cellars may confuse or be dismissed by those whose houses lack these features. Our city-dwellers may fail to develop a verbal distinction between jacuzzis and bathtubs, leaving many bathers driving themselves crazy trying to figure out how to turn on the jacuzzi jets in their ordinary tubs. Those who are familiar with staircases from sloped parts of the street, but live in one-story houses, may think talk of "internal staircases" is a rather desperate attempt to describe kitchen tables and chairs, and think those who say that internal staircases are really just the same as external ones have an odd notion of what qualifies as a close analogy.
It's a safe bet that knowledge of fire hydrants, stoplights, and cars in this city will progress a good deal faster and farther than knowledge of kitchen tubs, sinks, and lamps. Car mechanics will gain prestige as their discipline passes from strength to strength, while electricians and plumbers will find the situation increasingly embarrassing. Their studies may, indeed, attract a good deal of interest at the popular level, on the grounds that plumbing and electricity are important to people who want to live happy lives (e.g., to read books after dark, and to keep clean). But consensus will be elusive, and there will be a general feeling that while the car mechanics of today certainly know a lot more about their subject than the car mechanics of the past, the same cannot be said with anything like the same confidence of plumbers and electricians. Meanwhile, there will be some who, observing the way interior studies are always leading to bizarre myths, like the perennial claim that bathtubs have jacuzzi jets, or to insoluble disputes, like the one about whether internal staircases are more appropriate for walking on, like external staircases are, or whether they are merely platforms on which to eat food. Some critics may question whether there is any reason to believe that "houses" really exist at all, or, if they do in some sense exist, whether they are not reducible to streets. Some of these critics may be sufficiently sophisticated to scoff at the naive answer, "But I know houses exist. I live in one!"
Readers will have guessed that the houses are minds/souls. The streets are the domain of intersubjectivity. My point is that the domain of intersubjectivity is not coextensive with the domain of reason. We can reason from internal experience in much the same way as we do from our sensory acquaintance with the external world; indeed, knowledge from introspection is immediate and indubitable, while knowledge of the external world has-- as Karl Popper has noted-- an inevitably conjectural character. But any discussion has a bias in favor of the domain of intersubjectivity, and also, in the domain of intersubjectivity, the accumulation of knowledge is accelerated by efficiency advantages that result from division of labor and gains from specialization. I think this accounts for much of the interminable character of philosophical discourse. Philosophy has one foot in the domain of introspection and one in the domain of intersubjectivity. In the domain of introspection, it is hard to establish consensus. And the domain of intersubjectivity enjoys a natural advantage, greatly augmented in recent centuries by the advance of natural sciences. This empowers philosophers who try to abolish the domain of introspection and make the domain of intersubjectivity master (e.g., Daniel Dennett). Other philosophers, wiser though not more brilliant, carefully and painstakingly reclaim turf for the domain of introspection, but due to the nature of that domain, they can only persuade, rather than prove.