Yes, all those things. ("Symbiotic" doesn't quite get at it because it doesn't imply this kind of equality on both sides.) Based on my personal experience and

Yes, all those things. ("Symbiotic" doesn't quite get at it because it doesn't imply this kind of equality on both sides.) Based on my personal experience and observations of others so far, I think a large fraction of programmers experience unit tests as (a) taking more time to write than the code under test; (b) requiring much more code than the code under test; and (c) relied upon to be correct much more than the code under test. (To be clear, I'm talking about unit tests, not integration or system tests.) In order to understand what you meant by TDD, I needed to unlearn all these things, and I suspect others will need to as well.