A Facebook friend of mine saw some UFOs recently. In fact, he photographed a bunch of them and posted them on his Facebook page.

My husband, a serious amateur astronomer, took one look and said, nah, those are weather balloons.

My friend’s posts got a lot of comments. A few people had conventional explanations. Most agreed that this was proof of Something Out There. But nobody suggested that these lights were ghosts. Or divinities (major or minor, take your pick).

Imagine these lights in, say, 14th century England instead of 21st century America. Ghosts! faeries! angels! would have been the talk of the taverns. Not extraterrestrials or weather balloons.

We only see what we are prepared to see. Extraterrestrials or weather balloons in one culture, supernatural creatures in another. We fit things into the slots already prepared for them, and can’t take in the world any other way.

Yes, over time the categories shift; some things are added, some are subtracted; some combine and some separate. But the basic process, while it has fluidity, remains the same: we can only take in what is, in some sense, already there. We can’t help it. If we didn’t do this, our minds wouldn’t function at all.

The 13th century Chinese monk Wumen wrote “In the rice, there is sand.” Indeed. Most of the time most of our thoughts are reasonably accurate — that’s the rice. If you see a red light, stop. If you see a green light and nobody is barreling through the intersection, go. It’s so highly improbable that a red or green light in that shape and that position is anything other than a traffic signal that we can ignore the possibility. The model fits well enough.

But there is sand in the rice. Inescapably so. You are so pleased that Jerry Sandusky has taken a personal interest in your son. You are so lucky that Bernie Madoff invited you to invest with him. Your model of a child molester does not include a guy like Sandusky. Your model of a thief does not include a guy like Madoff.

We see what we are prepared to see. We don’t see what we’re not prepared to see. And it’s really hard to let our preconceptions go.

Not only is there sand in the rice, but it’s not clear that they are distinct. Nothing in this tremendously complicated system of categories we use to navigate our way is ever exactly right. Any image we have of thieves, murderers and child molesters will be both incomplete (missing some thief, murderer or child molester) and too expansive (including someone who is not a thief, a murderer, a child molester). Even traffic laws are ambiguous: Define “full stop.”

So my friend clung to his vision of extraterrestrials. And I clung to my certainty of: Nope, no extraterrestrials here. The world we live in is the world we make. Until, suddenly, it isn’t.