If you cannot precisely define the whole sample space . . .

Not disputing the statement but highlighting it as a catalyst for reflection.

From the Wikipedia entry for Probability Axioms, regarding the second axiom,

This is often overlooked in some mistaken probability calculations; if you cannot precisely define the whole sample space, then the probability of any subset cannot be defined either.

Probably related in some fashion to the inclination to make arguments based on apples-to-oranges comparisons.