The natural world is like a diamond – it has many sides. It can be scary (storms and earthquakes), welcoming (a cozy forest bower), inspiring (jaw-dropping mountain view), or isolating (a lonely empty beach). The Cay represents the natural world in all its many sides. The tiny island where Phillip and Timothy land is like a microcosm (mini version) of the larger world: both a loving home and a dangerous battlefield.
The Cay also asks us to think about the relationship between humans and the natural world. You'll sometimes find Theodore Taylor comparing or contrasting images of nature with human action. In this way, the book asks us to think about the link between the two. How are humans and elements of the natural world similar? How are they different?
We see different relationships to the natural world through the book's two main characters. Phillip's understanding of nature reflects his background: he's attended school and has some understanding of science. He can speak intelligently about volcanoes and coral reefs from his reading about them in books. Timothy, on the other hand, has no schooling and is very superstitious. He attributes bad luck to Stew Cat, for example. He does, though, have years of direct experience with nature and knows how to survive on his own in the wild. Both characters can learn something about the natural world from each other.
The Cay argues that we can learn about nature from books, but to really understand the natural world we must experience it firsthand.
Timothy gains respect for nature when he comes to understand that just like him, all of the animals on the cay are just trying to survive.