Your reason and the cause of your belief differ

We are fluent at producing justifications, and the fluency has nothing to do with whether the justification is the actual explanation.

Ask why someone prefers a particular brand of tea and you will get an answer immediately: the flavour, the price, something about the way it is packaged. The answer is sincere. It is also, quite often, constructed on the spot to fill the gap left by a question. The real story might be that their household drank it, that it was on the shelf at eye level for years, that a friend served it once when they were in a good mood. None of that is available to introspection. What is available is a plausible sounding reason, delivered with total confidence, because producing plausible reasons is something people are extraordinarily good at.

This matters because a reason and a cause come apart in an important way. A cause explains how you came to hold the belief. A reason is meant to justify holding it, and justification can be examined by anyone. If your reason is the real one, you can imagine it failing: if the tea turned out to be dearer than the alternative, would you switch. If the answer is a flat no, then price was never the reason, whatever you said. That test is uncomfortable to run on yourself, which is a decent sign it is doing something.

None of this makes beliefs formed by habit automatically wrong. The tea might be excellent. Plenty of things we absorbed without examining turn out to be correct, and rebuilding every view from scratch is neither possible nor sensible. The point is narrower: when you are asked to defend a view, notice whether you are reporting your reasoning or generating it. The generated version tends to arrive too fast and fit too neatly. Real reasons usually come with rough edges, and with at least one thing you concede on the other side.