A principle by which an equal amount of something will have less effect depending on how much has already been sampled. This works on all sorts of levels. Imagine chocolate. At first one small square of Cadbury's Dairy Milk is sufficient to get a kick. But after that, eating one at a time is not enough to get a similar kick. It has to be two the next time, and maybe even three the next. Until you feel sick.
More seriously it is also the principle which underpins the idea of increasing sex and violence in films. We like being titillated - but in order to be titillated we must see more sex or more violence than we did last time. The same amount will not have the same effect twice. Hence films get more sexually explicit and more violent. I'm not sure I agree with the conclusions which are usually drawn from this, but it's the way the reasoning goes.
This law explains why the halves of scone in a Devon cream tea have to have more and more jam and cream on them.