I’ve only just started reading Daniel Kahneman’s Thinking, Fast and Slow, but it’s already verified something I’d observed several times: France exhausts me.

I speak enough French that I can follow a conversation. My husband and his French friends are allowed to speak at full speed as long as they don’t expect me to say anything substantive — or if they speak some English, I’ll contribute my portion in that.

At the end of any day in which I’ve had to keep up in French, I’m zombie-tired. I’ve always explained it thusly: “I can speak French as long as I donate every available brain cell to it.”

Kahneman has my back. Basically, you have two mental systems:

System 1 operates automatically and quickly, with little or no effort and no sense of voluntary control.

System 2 allocates attention to the effortful mental activities that demand it.

For languages you speak fluently, you’re working in System 1. It didn’t take any work for you to read this sentence. In fact, you couldn’t not understand the words in the sentence. It happens at a level below awareness or control.

But I don’t speak French fluently. I know just enough that I can process French in System 2, where I’m spending an enormous amount of mental energy trying to keep up with the conversation.

Kahneman would argue that “energy” is the way to think of it:

System 2 and the electrical circuits in your home both have limited capacity, but they respond differently to threatened overload. A breaker trips when the demand for current is excessive, causing all devices on that circuit to lose power at once. In contrast, the response to mental overload is selective and precise: System 2 protects the most important activity, so it receives the attention it needs; “spare capacity” is allocated second by second to other tasks.

Either I can figure out how to get to the Louvre, or I can listen to Claire talk about her teaching job. I can’t do both.

It’s not me. It’s my brain.