What activities did George Berkeley think spirits perform?

For the Irish philosopher George Berkeley, a spirit is defined as a single, undivided, and active being. He categorized the activities of these spirits into two primary functions:1

• Understanding: This is the act of a spirit perceiving ideas.1
• Willing: This is the act of a spirit producing ideas.1

Berkeley distinguished these active spirits from the passive objects of the mind, which he identified as sensible ideas or sense-data. While human spirits perceive ideas, Berkeley also famously argued that God’s continuous thinking—or perceiving—is what maintains the existence of the physical world even when no human is present to observe it.21