The title of this blog is a nod to something the great physicist and Nobel laureate Richard Feynman once said. Here is a great clip one of his lectures where he talks about the difference between knowing and understanding. It gives a good insight into how a theoretical physicist can hold several different theories in their head to explain a phenomenon, but not know which is the correct explanation.
He also gives a great example of ancient Mayan astronomers who were able to predict astronomical phenomena very accurately but had no underlying theory for what the physical reality that produced these phenomena, e.g. they did not know what the moon was but they could accurately predict its phases.
Please excuse his assumption that physicists are men – he was very much a product of a different time.