- is one of the most complex concepts in Chinese and Japanese philosophy. It's been discussed in countless books through the ages, the most famous one of them being Tao te ching
, where Tao (Do) is presented as a kind of natural law for all of the universe.
The Japanese use of the term, such as in aikido, seems to be similar to the western concepts 'path' and 'quest'. I think it should be seen as through what you realize yourself and express your personal development in your life. A reasonable question to the Japanese is: "What is your do?" meaning "In what way do you express your progress in life?"
It's not important what do you choose, as long as you find one that allows you to express your own progress. It should fit you, and what it is that your life teaches you.
Well, much like an art.
Those who quit aikido quickly probably found that this quest was not theirs. So, they owe it to themselves to search for their do elsewhere.