They are as important as each other. However testers need to have a different mindset to developers and therefore should actively look for potential faults. If we only concentrate on positive tests (show that the system does what it should do) then we will potentially experience problems when the system goes live. If we only concentrate on negative tests (showing the system doesn’t do what it shouldn’t) then again we could potentially miss significant faults. However if we look primarily at breaking the system then we may find lots of faults (the what if scenarios) but we may not establish if the system is going to meet the users needs and requirements. A balance is needed with all three approaches.