Some may say that the purpose of testing is to validate assumptions made by business analysts, designers, or developers. To me, that is at best a potential side effect of testing—but not the goal. If you want testing to reveal important problems in the product, do not focus on validating assumptions (to do so would be more like checking; testing may include some checking). To foster discovery, excellent testing—like excellent science—seeks to invalidate assumptions.
To put it another way, it’s easy to check to show that something can work. As testers, we must probe, challenge and test the assumptions that might cause people to believe mistakenly that it will work. Those assumptions are where the risks live.