Because your brain gets information from your eyes telling it where your hands are. Eyes can’t see through solid matter so the only thing your brain has to rely on for the inside is nerve signals from an area. If no one ever opened up a human to check out what’s in there, you might be able to make some conclusions like, “I ate this berry and now the middle of my body hurts, it probably has something to do with the berry/eating”, but you wouldn’t know why exactly.
Of course it kind of depends on what he means by “knows”. The brain does know where your organs are and what they do because it has to send and receive signals from them, it’s just not your conscious mind because that isn’t the part responsible for those things. It’s not actively hiding the information, it just isn’t necessary for the “you” part of your brain to know, same as your speech centers don’t know you have legs.
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