This is the caterpillar of the Hermit Sphinx moth - which is a new one on me. It's enormous - it was at least 3 inches long, I'd say. And fat. There are some images online here. I'm a bit worried about him, because he was found by The Husband, who was clearing up around our frog pond, and under our big oak tree. And then he - The Husband - completely forgot about him and may well have squashed him inadvertently. I can only hope not.
At some point I might go out at dusk to look for the moths nectaring at my phlox. They apparently aren't attracted to light.