“When confronted with a problem, have you ever stopped and asked why five times? It is difficult to do even though it sounds easy. For example, suppose a machine stopped functioning:

1. Why did the machine stop?
There was on overload and the fuse blew.

2. Why was there an overload?
The bearing was not sufficiently lubricated.

3. Why was it not lubricated sufficiently?
The lubrication pump was not pumping sufficiently.

4. Why was it not pumping sufficiently?
The shaft the of pump was worn and rattling.

5. Why was the shaft worn out?
There was no strainer attached and metal scrap got in.

Repeating why five times, like this, can help uncover the root problem and correct it. If this procedure were not carried through, one might simply replace the fuse or the pump shaft. In that case the problem would recur within a few months.

To tell the truth, the Toyota production system has been built on this practice and evolution of this scientific approach. By asking why five times and answering it each time, we can get to the real cause of the problem, which is often hidden behind more obvious symptoms. 

Toyota Production System: Beyond Large-Scale Production by Taiichi Ohno, 1978. English Translation (Productivity Press, 1988) p. 17.