By the time Hayy is 21 years old, he has learned to use many forms of tools and technologies to survive. He makes clothes out of animal skins and sews them using hair, hemp, and fibrous plants. He stores food after observing birds do the same and keeps poultry to get eggs and meat. He uses animal horns and stone to make weapons and he domesticates wild horses so that he can hunt even animals that are faster than he is.

Hayy begins to contemplate how to categorize all the knowledge he has gained from sensory observation. While he initially divides everything into categories—seeing that all animals, organs, and types of beings are distinctive and different—he eventually begins to notice connections and patterns. This causes him to conclude that the many different components of the world are actually all one. He first unifies animals into distinctive species, then into the entire animal kingdom, and then does the same for plants. Regarding inanimate bodies, Hayy divides them based on their physical properties into the elements of earth, water, fire, and air. He notices how all material things have certain traits that unite them into categories such as hot, cold, heavy, or light.