How did John Locke explain the difference between sensation and reflection?

In his An Essay Concerning Human Understanding, John Locke identified two distinct sources for all human knowledge: sensation and reflection.123

• Sensation refers to the experience of external objects through the senses. It provides the mind with simple ideas derived from sensory input, such as the color of a rose, the sound of a bell, or the taste of salt.12
• Reflection refers to the mind's introspective awareness of its own internal operations. It is the process by which one becomes conscious of one's own thinking, including states such as happiness, sadness, or the act of having a specific sensation.123

Locke argued that these two sources provide the "materials" of knowledge, which he called "ideas". While he distinguished between these two origins, he emphasized that both are necessary for the mind to acquire the building blocks of conceptual experience. These ideas are not knowledge in the strict sense themselves, but they serve as the essential foundation upon which the mind builds complex concepts through the processes of combining and compounding.124