Principles are ways of successfully dealing with reality to get what you want out of life.
Ray Dalio, one of the world’s most successful investors and entrepreneurs, cites principles as his key to success.
If you didn't make an expectation clear, you can't hold people accountable for it not being fulfilled. Don't assume that something was implicitly understood. Common sense isn't actually all that common--be explicit. If responsibilities keep falling between the cracks, consider editing the design of your machine.