Soft systems are systems that include as part of the system human participants whose various interpretations and responses help regulate the workings of the system.

Hard systems are systems that do not include an interpretive human element and are assumed to be regulated entirely algorithmically.

Hard systems are engineered.

Soft systems are designed.

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The signature error of the 20th Century was to mistake soft systems problems for hard systems problems, and to attempt to engineer solutions where design was needed.