The main themes in the poem "The Castaway" are the power of nature, the power of man over nature, and the cyclical nature of life.
Throughout the poem, the natural world is personified. For example, the speaker says that "the surf tires of its castles like a child," and the sand, "Bored, shifts its dunes." Personifying nature in this way emphasizes its power. It's as if the natural world has a will of its own and can be emotionally volatile just like a person might be.
However powerful the natural world might be, the speaker in the poem seems to derive pleasure from exercising power, albeit temporary and futile, over nature. For example, the speaker cracks a sea-louse and in doing so considers himself "Godlike, annihilating godhead." This is in direct contrast to the speaker's appreciation, elsewhere in the poem, of the harmony and interdependency that exists between man and the natural world. In the seventh stanza, for example, the speaker, in a meditative mood, reflects that all people "end in earth, from earth began."
Overall, however, it seems as if the speaker has a rather hostile attitude toward the natural world. The speaker describes "the rage with which the sandfly's head is filled," "the dog's faeces" drying in the sun, and, tellingly, the "ripe brain rotting like a yellow nut." This last quotation perhaps indicates why the speaker seems to have a negative attitude toward the natural world. The natural world, with its indifference towards decay and death, perhaps reminds the speaker that he too must decay and die. His brain must inevitably rot, just as everything within nature must, and nature will be as indifferent to the speaker's death as it is toward every other death.