They say the eyes are the window to the soul, and I think, especially in this case, it's true. If you look at a picture of Biden from 2019 or earlier, especially from 2016 or earlier, you can see there is an intensity behind his gaze.

If you look at him now, he has a thousand-yard stare. He's just looking ahead, but seeing nothing. The intensity that was there in previous years is gone. His eyes are just empty. There were flashes of his old self, but not in weeks, maybe months.