Driving to work, I stop at the "tee" and make the left. But there is a pothole right there that's hard to miss. So I got in the habit of looking for the edge of the pothole and staying to the left of it, every morning.
And then one morning I didn't see the edge. I thought it odd that I missed it. The next day I noticed they patched the pothole. So, the odd thing was not that I didn't see the pothole, but that it was gone, and I didn't even notice!
But I wasn't looking for the pothole, you know? I was looking for the edge.
That minor event reminded me of the Darwin story related by Lionel Ruby in How the Scientist Thinks. I wrote of it before:
Darwin and a fellow scientist were searching for fossils in the north of England. They were not aware of the glacial theory at the time. Years later Darwin revisited the area, and he was now astonished to discover how clearly marked were the glacial ridges on the rocks. He had not noticed them on his earlier visit because he was not looking for them.... Darwin was able to appreciate the glacial markings only after he became aware of the glacial theory.
We see -- or maybe, don't see -- what we're looking for. But we can easily overlook things we're *not* looking for.
We know there is a relation between money and inflation. So that's what we see. That's what we see even when it isn't money that is causing inflation. That's what we see, even if we have to jigger the numbers like Friedman did to see it.
But if, for example, there is also a relation between credit use and inflation, and we are not looking for that relation, then we are very likely to overlook it.