What do I mean by “foolishness”?
"Take no thought for your life, what you may eat and drink...
be a living sacrifice...
turn the other cheek...
go the extra mile...
love your enemy."
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What I mean by “foolishness” is a life radically different than conventional wisdom and contrary to worldly values, and that is to live on earth as if we are already in heaven.  
It is being separated out from our self-interest and desires in order to be in loving service only to the will of God in caring for other souls.  It is no longer thinking of ourselves, others, and situations from a worldly point of view.