[3] The key idea of the method is to encourage the troubleshooter to avoid assumptions and logic traps and instead trace the chain of causality in direct increments from the effect through any layers of abstraction to a root cause that still has some connection to the original problem.
In this example, the fifth "why" suggests a broken shelf foot, which can be immediately replaced to prevent the reoccurrence of the sequence of events that resulted in cross-threading bolts.
[4] In this case, the factory may need to add a process for regularly inspecting shelving units for instability, and fixing them when broken.
Gottfried Wilhelm Leibniz used iterative why questions in his letter to Magnus von Wedderkop in 1671, in which he applied elements of argumentation that he later used to solve the question of theodicy: The modern technique was originally developed by Sakichi Toyoda and was used within the Toyota Motor Corporation during the evolution of its manufacturing methodologies.
Teruyuki Minoura, former managing director of global purchasing for Toyota, criticized it as being too basic a tool to analyze root causes at the depth necessary to ensure an issue is fixed.