When I was growing up, I was of the opinion that everyone was just interpreting reality in the best way they could, notwithstanding those evil brainwashed Christian commahippajewanazies.
Since no one felt the need to disabuse me of the notion, I assume you did too. Now, however, you will be treated to a full analysis of the real situation.
First, let me say that I'm aware of the irony and hubris of this statement, and will be slightly chuckling to myself for this whole essay. Second, I am aware of how this idea applies to me.
Specifically, while I haven't found a school of thought that I follow well, I've borrowed heavily from several, such as logical positivism, capitalist anarchy, Buddhism, and what I now call Universalism.
The stage is set. The actors walk into the limelight, and they ask each other, "Are people really coming up with their own ideas about the world?"
Are they at least selecting from the most cogent ideas available from a board of articulated and snappily dressed intellectuals?
In short, obviously not. Notwithstanding this fact, people carry on as if their ideas were really theirs. An astounding achievement, that is. I wish I could convince people that my ideas were really theirs. I'd have an army in no time and take over the world.
The problem with this, of course, is that it already happened. Specifically in Massachusetts, but I digress. (On a blog, no less!)
In reality, most people are members, in whole or in most, of some intellectual dynasty. Christian, atheist-Universalist, Libertarian, or something. What's more, each of these dynasties have either one person or a small cabal that made up their main ideas, possibly their axioms, which have passed unchanged through time.
For instance, the idea of souls that we now hold today, whether we agree or disagree with the notion, would not have bothered Plato at all. He's the dude who came up with the basic idea, at least as far as our records show. Descartes later added some refinements, which we now call Dualism.
Platonic ideals - entities existing 'above' this plane of existence, perfect and unchanging, do not appear as far as I know in history before Plato, and the idea of souls, far from being 'obvious,' is completely dependent on this idea. While uncertainty is great when we're talking about ideas this old, the basic point stands: nearly every widely held idea started as a kooky thought in the head of some weird old guy.
In fact, a truly well educated person wouldn't even bother talking about their ideas at all. None of this "Well, I believe that..." nonsense. Instead, they'd just have a list of problems, with an attached list of philosophers. Getting to know such a person would go something like this:
"Who are you on race relations?"
"Martin Luther. You?"
"Oh, Malcom X, definitely. What about the God issue?"
So the hypothesis:
Ideas spread by presenting themselves, received ideas, as created, and usually in blocks or what I call 'suites.' As in, "the full office software suite." Or, "the full Democrat suite, with some customizations on finance."
It may not always have been this way. For instance, when Christianity was dominant, certain things were called sacrosanct and were privileged. They were labelled explicitly. Now, certain things are still holy, but go out of their way to present themselves otherwise, usually as self-evidently true, and yet still questionable. It may be, then, that holding other people's ideas as your own is an innovation, or it may not be.
The suites will warp logic in the suite user. If it were not so, the suites would rapidly decay on contact with either evidence or competing or more useful ideas. My evidence implies that the confirmation bias and identity biases are the main culprits. The suites encourage the user to say, "I am a progressive," and thus to take progressives conclusion, lest they allow insults to their personality. Having taken the conclusion, they build up evidence below it.
The suites are usually not full personal ideologies. The person who gets all of their understanding from any one source is rare. Usually, they are sets of related ideas from a small group of thinkers.
Take a person self-labelled as a freethinker. Anyone likely to take offense to statements like, "You just believe that because you're a progressive."
Get to know what they think. That should be enough, as you'll be able to clearly see which dynasty they fall into.
When I did the test, I found it's really amazing to see ideas just pop out as clearly the work of some weird old guy. Consider how often the random person finds themselves incapable of remembering what you said even last week, and combine that with a widespread distrust of authority. It's incredible to see how strongly influenced they are by well-known philosophers, like Kant or Hobbes.
Did they ever read Critique of Pure Reason? Did any of their friends? Their teachers? No. Yet, even a casual perusal of La Wik's article on Kant finds several ideas that are in common parlance. Similarly, lots of ideas that are supposedly just common sense are actually exact copies from Hobbes and Hume.
In fact, reading even Shakespeare naively, such as during the high school English classes, one tends to say, "What cliched hackwork!" But of course, this is just because so many people wanted to imitate the Bard, followed by imitators of the imitators who recognized a nice turn of phrase.
Similarly, reading the ideas of the famous philosophers, one is tempted to use that ever present 'obvious!' to describe their ideas, to say, "But certainly this is simply a recording of what people were already thinking." This is false. Even if it were half true, or even 90% true, these philosophers got their ideas seeded into the English consciousness. That is exactly why they're considered great.
And now, poor ignorant us, think we're coming up with our own ideas. Ideas we will, too often literally, defend to the death. Pathetic.