What does the statement that testing demonstrates the presence of defects, not their absence, mean?

Testing aims to find defects. The process of executing tests and analyzing results is aimed at identifying cases where the system's behavior differs from the expected (specification).

However, even passing tests do not guarantee the absence of defects. There is always a probability that:

• Test coverage is incomplete.
• Not all usage scenarios have been tested.
• Defects manifest only under certain rare conditions or combinations.
• The specification itself contains inaccuracies or is incomplete.

Thus, successful testing shows that the system behaves correctly in tested scenarios but does not prove the absence of defects in untested areas. Testing is a defect search, and its success is measured by the number of defects found and fixed, not by their total absence.