I'm currently reading The Sense of Style, partly because everyone else seems to be reading it, and partly because it's so good I have trouble putting it down. The book is a contemporary style guide, combining linguistics, cognitive science and writing convention in a compendium of advice on writing well. Linguist, writer and cognitive scientist Stephen Pinker is obviously well-placed to write such a book, and I'm glad he has.
Aside from being an interesting and useful guide on forming pleasant English, this book has also given me something else: it's allowed me to capture an example of my own bad behaviour out in the wild. You see, I think I know about grammar, but I don't. Not really. Beyond secondary school I have no formal education in the subject, but I've read enough around it to feel like I know what I'm talking about. This is useful, because I have an in-situ example of what this behaviour feels like from the inside.
It's not that I don't know much about the subject. Far from it. Compared to Average McStreetPerson I know an awful lot about grammar and linguistics, but that's a lousy scale to hold oneself against. Someone who reads a lot of popular science knows much more about physics than Average McStreetPerson, but that doesn't mean they know a lot about physics compared to the field of academic inquiry known as physics.
There are a few features of Pinker's book that alerted me to my imaginary expertise in grammar, most notably his talk about contemporary theories of grammar overthrowing incumbent ideas I thought of as canonical. In the abstract I was dimly aware that people must be doing modern research into the subject, but I could't have told you what any of it was.
I can't remember where, but I distinctly remember reading about some research on how humans have trouble distinguishing between familiarity and knowledge. This is presumably responsible for the experience of looking over what you've just been revising, and feeling like you know it without actually having retained anything. I wonder if imaginary expertise is like this. I'm familiar enough with, say, verb forms to follow a discussion about them, but I don't have command of them. My familiarity is reinforced every time I follow such a discussion, but my command faculty is rarely challenged, as it would be if, say, I took an exam.
If this is the case, it gives a useful rule of thumb, which in retrospect seems obvious: take tests on subjects and see where you fail. The less obvious aspect of this rule of thumb is presumably to avoid questioning the need to take a test, or to make excuses as to why they're not necessary.