The ability to associate imaginary physical sensations with abstract concepts is called _synesthesia_. Some people see letters in colors. Others see the days of the week as if they were positioned in the space around them. There’s a widespread belief that synesthesia is rare and associated with certain mental conditions. In reality it’s a universal phenomenon and a core building block of human cognition. Here’s a little test to see if you’re capable of synesthesia: looking at the word _chocolate_, are you able to sense a sound, a color, a taste? Looking at “999,999,999,” do you get the feeling of something large? What is rare, and what our culture doesn’t push you to do, is to be aware of your capability for synesthesia and to try to develop it systematically. Secret math is a mental yoga whose goal is to retake control over our ability for synesthesia.
