Despite being a brief novel, The Cay includes many important ideas throughout the text. One such idea or theme is focused on racism. Phillip is an incredibly racist child at the beginning of the book, and he speaks quite abusively to Timothy on multiple occasions. To be fair, Phillip also cruelly talks to his own mother; however, Phillip's insults against Timothy come from his belief that black people are inferior. Phillip will learn that his beliefs are wrong, and he will come to love and appreciate Timothy. The two characters develop an incredibly strong bond while on the island, and it is heartbreaking when Timothy dies.
The book also includes powerful messages about perseverance. Timothy and Phillip are in a tough situation. The small cay that they are on is not overflowing with water sources or food sources. They have to work hard to survive, and Phillip has to learn to do it all without the use of his sight. He initially feels that he can't do anything while being blind, but he learns through hard work and dedication enough skills to survive on the island without Timothy's help.
Finally, the book teaches readers that having hope is important. Timothy knows that they are not on an island near a busy shipping lane. He knows their chances of rescue are slim, yet he never allows himself or Phillip to wallow in self pity. Hope is something that both characters cling to because it is a motivator to keep on surviving.