The well-known discussion of the principles of experimentation, illustrated by a taste-testing problem, in R. A. Fisher's Design of Experiments, is the basis of this expository paper. The notion of a hypothetical population of identical experiments is defended. It is argued that attention must be paid to non-null cases (in testing theory) if a satisfactory probabilistic model for sensory sorting tests is to be built, and if the efficiency of various experimental designs is to be considered. Finally, some remarks are made on the role of randomization, and on the problem of “inexact” acceptance regions in discrete distributions.