I'm loving this series, @spectrumecons!

Once we learn to see things in terms of complementarity and substitutability, we can see so many other phenomena in a new light:

Personally, I like thinking about beliefs and inferences as mental tools that can be analyzed in this way. 'X' and 'not-X' are strong (maybe even perfect) substitutes for each other. 'If P, then Q' means that Q is a strong (maybe perfect) complement to P. But this does not necessarily mean that P is just as good a complement to Q.

Inter-personal power can also be analyzed in this way: A powerful person in one that functions as a strong complement or pre-condition to other people's well-being but has few if any substitutes to challenge his/her position.

While my ideas aren't completely clear on the subject, it seems like substitutability is simply another word for "zero-sumness" while complementarity means "non-zero-sumness"... But my convictions on this point and quite negotiable. (I'm open to substitutes!)