Would you know it if your brain was not functioning correctly? Perhaps; it depends on exactly what problem it was having.
Would you know if you had forgotten something? How could you if you have forgotten it? Wouldn’t simply knowing that you knew it allow you to know it? Again, it depends on the knowledge. It’s possible to know that you used to know how to integrate a calculus problem, but be unable to do so anymore because you’ve forgotten how. You have lost the primary knowledge, but retained the meta-knowledge, that is, knowledge about other knowledge. On the other hand, you could know that you have an appointment tomorrow, but forget it. There is no meta-knowledge involved in this however because the mere act of knowing that you know that you have an appointment is itself the primary knowledge: that you have an appointment. The closest thing to meta-knowledge in this case would be knowing that you had to do something, but cannot remember what, but that is too generic and applies to pretty much all knowledge one has.
What about more significant damage? What if you incur Alzheimer’s disease? Would you know it? How could you since the disease affects the fundamental ability to think?
These problems arise because the brain is our primary (and only) method of thinking about things, including our brains themselves. If the brain gets damaged, how can it think about itself? If you sustain some brain damage resulting in a loss of cognitive ability, how can you know it if that requires the brain to know and understand it?
This is an issue with other complex things too, such as electronics. If a cup falls on the floor, you can pretty easily and thoroughly check it for damage, as long as it isn’t broken or chipped and can still hold water, it’s fine. But what if there is a power-surge that hits your TV or computer or something? What if you drop your mobile-phone? Those things are so complex that it could be very difficult to detect damage. It might turn on and look fine, but what if one of the speakers isn’t working? What if the Bluetooth module isn’t working? What if some other part that you don’t normally use is damaged and so you don’t detect it for some time? What if a single transistor in a processor is damaged that might not show up for a very long time because the circumstances for it be used and fail are uncommon?
For that matter, how can the manufacturer even be sure? When a factory produces something as complex as a microprocessor, how do they test it thoroughly enough to be sure that the billions of transistors are all correct? 🤨
Can you ever trust anything to be in good condition? 🤔
There’s actually a secondary cause as well. The brain is very complex, so it’s entirely possible for one or more parts to have damage but go unnoticed because they’re not trying to be used, so it seems like everything is fine. It’s like with complex things like a computer; if there’s a static discharge that zaps the computer, simply turning it on won’t necessarily tell you if it’s okay since it might turn on and boot up and seem fine for a bit, but still be damaged in a part that you haven’t used like the audio card or the Bluetooth radio or something else. Only if you actually try to use those would you then realize that there’s a problem, but it could even be something more insidious like a specific transistor in an IC that affects a specific function in a specific module so you might not notice a problem for a long time, or even possibly ever, but it’s still there. The brain is the same way; a specific memory or ability or something might be damaged but you wouldn’t know it until you actually try use it.