A fundamental stance taken in human-centered computing is that information processing devices must be thought of in systems terms. At first blush, this seems self-evident. However, the notion has a long history, and not just in systems engineering. In this new age of symbiosis, machines are made for specific humans for use in specific contexts. The unit of analysis for cognitive engineering and computer science is a triple: person, machine and context The triples rule asserts that system development must take this triple as the unit of analysis, which has strong implications, including a mandate that the engineering of complex systems should include detailed cognitive work analysis. It also has implications for the meaning of intelligence, including artificial intelligence.