1. 射影幾何の定理における点と面の役割の互換性(the interchangeability of the roles of points and planes in the theorems of projective geometry)
2. いくつかの特性は波動説、他のものは粒子論により最も良く説明されるという事実により特徴付けられる物質と電磁放射の特性(the property of matter and electromagnetic radiation that is characterized by the fact that some properties can be explained best by wave theory and others by particle theory)