A helpful metaphor for the seven testing principles of ISTQB is that of a fisherman trying to catch fish. Just as a fisherman must carefully select the right bait, location, and time of day to see the fish they want, software testers must choose the right testing strategies, tools, and techniques to find defects in the software they are testing.
1-Testing shows the presence of defects
Just as a fisherman casts a line to see if any fish are biting, software testers perform tests to see if any defects are present in the software.
2- Exhaustive testing is impossible
A fisherman cannot possibly catch every fish in the sea, and software testers cannot test every possible combination of inputs and scenarios. Instead, they must focus their efforts on the software's most important and relevant parts.
3-Early testing
Just as a fisherman sets out early in the morning to catch the most fish, software testers should begin testing as early as possible in the software development lifecycle to see defects before they become more costly and difficult to fix.
4-Defect clustering
Just as a fisherman knows that certain lake areas are more likely to have more fish, software testers should focus their testing on the software parts that are most likely to contain defects.
5-Pesticide paradox:
Just as a fisherman knows that fish may get used to a particular type of bait and become harder to catch, software testers should regularly review and update their testing strategies to find new defects.
6-Testing is context-dependent
Just as a fisherman knows that different fish require different types of bait and lures, software testers must adapt their testing strategies to fit the specific context of the software they are testing.
7-Absence-of-errors fallacy
Just as a fisherman knows that the absence of fish on the line guarantees no fish in the lake, software testers should remain vigilant and open to the possibility that defects may exist in the software even if they have not been found yet.