Buddhists have an idea that you can judge a man by the things he repeatedly does

They believe that your essence, or soul, is determined by repetition.
By repeatedly *doing* certain things, those things come to define you.

Modern science has proven this in a way.
The pathways between neurons grow stronger through repetition,
essentially turning a dirt path into an interstate.
This is similar to shadow image burn on older TVs,
and "muscle memory".