A man may imagine he understands something, but still not understand anything in the way that he ought to. (Paul of Tarsus, 1 Corinthians 8:2) Calling this a 'practical theory' may require some explanation. Theory and practice are often thought of as two di?erent worlds, governed bydi?erentideals, principles, andlaws.DavidLorgeParnas, forinstance, who hascontributedmuchtoourtheoreticalunderstandingofsoftwareengineering and also to sound use of theory in the practice of it, likes to point out that 'theoretically' is synonymous to 'not really'. In applied mathematics the goal is to discover...

A man may imagine he understands something, but still not understand anything in the way that he ought to. (Paul of Tarsus, 1 Corinthians 8:2) Calling this a 'practical theory' may require some explanation. Theory and practice are often thought of as two di?erent worlds, governed bydi?erentideals, principles, andlaws.DavidLorgeParnas, forinstance, who hascontributedmuchtoourtheoreticalunderstandingofsoftwareengineering and also to sound use of theory in the practice of it, likes to point out that 'theoretically' is synonymous to 'not really'. In applied mathematics the goal is to discover...