What are the four underlying principles of the Toyota Production System?

The four underlying principles are: 1. all work must be completely specified as to content, sequence, timing and outcome; 2. every customersupplier connection must be direct, unambiguously specifying the people involved, the form and quantity of goods and services, the way requests are made by customers, and the expected time in which the requests will be met; 3 . the pathway for every product and service must be simple and direct; and 4. any improvement to the system must be made in accordance with the scientific method, under the guidance of a teacher, and at the lowest possible level.