I'm not a great chess player, but that is one thing my dad taught me.
Later, a friend of mine gave me this book, during all the Ron Paul stuff: Every move must have a purpose.
It is excellent advice, in chess and in life.
Looks like it is out of print, but still available for a penny on Amazon.
To be mean is never excusable, but there is some merit in knowing that one is; the most irreparable of vices is to do evil out of stupidity. - C.B.