In the eighteenth century, the philosopher George Berkeley argued that reality was all in our minds. Samuel Johnson had no patience for this idea; he declared, “I refute it thus!” and kicked a stone. Two centuries later, the poet Richard Wilbur wrote a rejoinder:

Kick at the rock, Sam Johnson, break your bones:
But cloudy, cloudy is the stuff of stones.

“It’s not real, but it doesn’t matter,” Slater said, watching me. “In some sense, it’s a real experience.”