Syllabus Reference: 1.3.1 — Testing principles

Source: ISTQB CTFL v4.0 Sample Exam A, Question #3

You have been assigned as a tester to a team producing a new system incrementally. You have noticed that no changes have been made to the existing regression test cases for several iterations and no new regression defects were identified. Your manager is happy, but you are not. Which testing principle explains your skepticism?