It can't float in mid-air, particularly if the material involved is sand, mud, or molten rock.
The principle of superposition therefore has a clear implication for the age of a vertical succession of strata.
A common form of criticism is to cite geologically complicated situations where the application of radiometric dating is very challenging.
They are applied by geologists in the same sense that a "null hypothesis" is in statistics -- not necessarily correct, just testable.
In the last 200 or more years of their application, they are valid, but geologists do not assume they are.
An early summary of them is found in Charles Lyell's .
In no way are they meant to imply there are no exceptions.