And clearly, debt by itself doesn’t matter — it’s how debt leads to other economic events that matters.
What matters most isn’t a correlation, it’s the mechanism by which causes bring about effects that are most important. Every wag who reads a study he disagrees with immediately resorts to the trope of “correlation isn’t causation.” And sometimes it’s true. It’s important to be very clear about what claims we’re actually making. Establishing causality is difficult, but also very important. When people do achieve it (and they occasionally do), it’s a real addition to our knowledge.