No child but must remember laying his head in the grass, staring into the infinitesimal forest and seeing it grow populous with fairy armies.

Stevenson evokes a near-universal childhood experience: the ability to turn close observation of nature into imaginative spectacle. With one’s face in the grass, the “infinitesimal forest” (blades, stems, insects) becomes a whole world, and the mind supplies “fairy armies” to populate it. The line suggests that imagination is not an escape from reality but a way of intensifying it—seeing more in what is already there. It also carries a note of nostalgia: adulthood often loses this spontaneous animating power, so the memory becomes a touchstone for innocence, wonder, and the creative origins of storytelling.