"Touchstone" refers to a fundamental or quintessential part or feature of something, serving as a standard or criterian for evaluation. It can also refer to a stone used to test the quality of goods.

In a broader context, it signifies a benchmark or yardstick against which other things can be measured or assessed.

The historical term milestone is still used today, even though the “stones” are typically metal highway location markers and in most countries use kilometres and metres rather than miles and yards. Also found today are more closely spaced signs containing fractional numbers, and signs along railways, beaches and canals.

“Remember to celebrate milestones as you prepare for the road ahead.” — Nelson Mandela

“Pain is the touchst ...