Why did George Berkeley believe that extension and motion are ideas?

George Berkeley, an Irish philosopher, argued that extension and motion are ideas because they are sensible qualities that cannot exist outside of a perceiving mind.12

Berkeley rejected the distinction between primary qualities (such as extension and motion) and secondary qualities (such as color and taste), which other philosophers like John Locke had maintained. Berkeley’s reasoning included the following points:12

• Direct Perception: He argued that all sensible objects are things we perceive by sense, and because we perceive only our own ideas, it follows that all sensible objects—including those characterized by extension and motion—are ideas.12
• Nature of Extension: He contended that extension must be understood as either visual or tactile. Since both visual and tactile experiences are forms of perception, they cannot exist independently of a mind.3
• Rejection of Material Substrate: Berkeley denied the existence of an imperceptible "material" substance or substrate that supposedly "holds" these qualities. He argued that since all knowledge is derived from perception, the notion of an unperceived, material object is contradictory.32

Ultimately, for Berkeley, to be is to be perceived (esse est percipi), meaning that physical objects are simply collections of these sensible ideas.12