Which of Kawakami's rules is perhaps the most crucial differentiator between Chindōgu and a standard commercial invention?

Answer

It cannot be patented or sold.

The text states that if an invention can be patented, mass-produced, or sold as a commercial success, it ceases to be Chindōgu because its value lies in its unuselessness.

Which of Kawakami's rules is perhaps the most crucial differentiator between Chindōgu and a standard commercial invention?
inventiondesignconceptcreatorchindogu