G. K. Chesterton wrote this in Orthodoxy in 1908, a book he published in his mid-thirties as a half-autobiographical, half-theological defense of a Christianity most of his contemporaries had stopped taking seriously. Chesterton himself was a famously large man, which gave the line about lightness a slight extra weight it would not otherwise have had.

The sentence is a pun doing real work. Taking oneself lightly is not the same as not caring. It is the practice of refusing to treat one's own dignity as load-bearing infrastructure. People who cannot laugh at themselves accumulate a ballast of seriousness that eventually keeps them on the ground. Chesterton, who believed in big things, considered self-importance the cheapest way to lose altitude. The angels, he suggests, manage the trick by not performing gravitas.

A Persian sprawled upside-down on a bookshelf, belly up, all four paws pointing in different directions, is demonstrating this with unusual commitment. The cat has no reputation to protect in the pose. It has only abandoned seriousness, and the abandonment is the whole joke, and the joke is somehow also the point. We tend to lose this ability around age eleven and spend the rest of our lives trying to buy it back. The cat, it should be noted, never paid for it.