Seven Principles of Testing

According to the ISTQB there are seven principles of  testing. They are ,

• Testing shows presence of defects: Testing can show that defects are present but it is not the proof of correctness of the software product. Testing cannot find out all defects there can be many bugs remain in the software.
• Exhaustive testing is impossible: Testing all combinations of the software is not possible. It focus on critical and most important business functionalities which means it prioritize risk clustered areas.  Testing involve in mitigating the risk of the application. 
• Early testing: Testing should be start with as much as early actually it should start withe strat of SDLC (software development life cycle). 
• Defect clustering: A small number of modules or areas contains most of the defects found during the pre-release testing or shows the most operational failures. 
• Pesticide paradox: If same set of test cases run again and again but won't able to find bug further then need to revise and review test cases regularly also need to write test cases to exercise different paths of the software.  
• Testing is context depending: In real world testing is depend on the context. For an example the way of testing banking application and human resource application is different because banking application is much concern on security and HR application does not
• Absence – of – errors fallacy: If the software built is unusable and does not meet the user’s needs and expectations then finding and fixing defects does not help. Therefore first attempt of a test engineer should be to check the built and expected are match each other. 

Reference : http://istqbexamcertification.com/what-are-the-principles-of-testing/