His theories were deceptively simple, yet remarkably profound.

profound 是形容词，由前缀 pro-（表示"向前"或"完全"）和词根 found（来自拉丁语 fundus，意为"底部"）组成，字面意思是"达到底部的"，引申为"深刻的、深奥的"。在这里形容理论具有深层次的内涵和意义。