As with all quantum devices, a qubit is a delicate flower. If you so much as look at it, you destroy it. - Hans Christian von Baeyer

The problem of defining exactly what is meant by the signal velocity, which cropped up as long ago as 1907, has not been solved.

As every bookie knows instinctively, a number such as reliability - a qualitative rather than a quantitative measure - is needed to make the valuation of information practically useful.

The solution of the Monty Hall problem hinges on the concept of information, and more specifically, on the relationship between added information and probability.