We understand the world through our senses, which are SIGHT, SOUND, TOUCH and SMELL.

Geometry and color are seen. 

Sound is heard. 

Texture and temperature are felt. 

Odor and taste are smelled.

Any sensation can bring to mind impressions of other sensations. We can imagine the taste or smell of purple, or what sharp looks like.

The internal world is made of thought, which are the impressions of these sensations, and the external world is made of particles in motion.

Most memories, ideas, concepts, etc. are made up of multiple sensations. A past lover will be seen, heard, touched and smelled in the mind as you please.