Aquinas thinks that natural efficient causes can act through the active powers of substances distinct from themselves. Aquinas identifies two ways in which an efficient cause can operate through another cause’s power, namely as an instrumental cause or as a secondary cause. The chapter discusses Aquinas’s basic understanding of instrumental causality and secondary causality. Instrumental causes are employed by another cause, called a principal cause, to reach its end. In acting for a higher end, the instrument acts through the principal cause’s power. Secondary causes are like instruments insofar as they cannot act unless a higher cause exercises its power. However, secondary causes differ from instruments insofar as they act for their own ends. The chapter discusses Aquinas’s examples of instrumental and secondary causality in the natural world. Aquinas uses of the notion of instrumental causality to understand how higher-level natural powers, such as the nutritive power, employ the actions of elemental powers, such as heat, to reach their ends. He regards terrestrial causes as secondary causes that act through the power of the heavenly bodies.