In foundation level syllabus you will find the main basic principles of testing, Which of...

One of the basic principles of testing is that for a software system, it is not possible under normal conditions to test all input and output combinations because they are too many and too complex to cover within a reasonable time and budget. Therefore, testers have to select a subset of inputs and outputs that are representative and relevant for testing based on criteria such as requirements, risks, priorities, etc. This principle implies that testing cannot prove that a software system is defect-free or has a certain level of quality because there might be some untested inputs or outputs that could reveal defects or failures. Testing can only provide information about the quality and risk level of a software system based on the observed behavior and results under certain conditions and assumptions. You can find more information about the basic principles of testing in A Study Guide to the ISTQB® Foundation Level 2018 Syllabus, Chapter 11.