"When you know both yourself as well as your competition, you are never in danger. To know yourself and not others, gives you half a chance of winning.  Knowing neither yourself or your competition puts you in a position to lose."

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A new world of complex relationships and feelings opens up when the peer group takes its place alongside the family as the emotional focus of the child's life. Early peer relationships contribute significantly to the child's ability to participate in a group (and in that sense, society), deal with competition and disappointment, enjoy the intimacy of friendships, and intuitively understand social relationships as they play out at school, in the neighborhood, and later in the workplace and adult family.

— Stanley Greenspan