“You don’t know what you don’t know.”
This phrase isn’t meant to make anyone feel small. It doesn’t mean “you should know,” or “everybody else knows this,” or even that “the guy writing this knows.” This phrase is very democratic… it applies to each of us (particularly the guy writing this).
What it means is that, if our brains are libraries, even big ones, there’s only so much information that can fit inside. We may know the next 10, 100 or 1,000 books we want to add to our mental Alexandria, but we can’t want (or even hate) the book that we don’t know exists. The same is true of MRI safety.