The resulting diagram "is intended for discussion more than a set of hard and fast divisions." Fifth, he re-classifies engineering knowledge itself. Knowledge generated by engineering may normally be categorized by phases such as design, production or operations. Another way to think about engineering knowledge categories is descriptive knowledge, prescriptive knowledge and tacit knowledge. He adds Gilbert Ryle's terms "knowing that" and "knowing how" to illustrate the aim of each knowledge category. "Knowing what or that" to do in engineering is a mixture of descriptive and prescriptive knowledge. "Knowing how" to do it is a mixture of prescriptive and tacit knowledge. Thus, these case studies show the need for all three kinds of knowledge in engineering. Finally, he posits a variation-selection model for knowledge growth. At all levels of design hierarchy, growth of knowledge acts to increase the complexity and power of the variation-selection process by modifying both the mechanism for variation and expanding the processes of selection vicariously. Variation and selection each add two realistic principles for the advancement of technology: "blindness" to variation and "unsureness" of selection. Vincenti concludes that our blindness to the vast potential in variations of design does not imply a random or unpremeditated search. A blind person in an unfamiliar alleyway uses a cane to provide information to explore the constraints in an intentional way without having any idea where the alleyway leads. Likewise, engineers proceed in design “blindly” in the sense that “the outcome is not completely foreseeable” thus the “best” potential variations are in some degree invisible. As a result, finding high functioning designs is not the norm. He notes, “from the outside or in retrospect, the entire process tends to seem more ordered and intentional—less blind—than it usually is.” However, Vincenti uses the differences between the Wright brothers and the French to show there is a range in how we manage blindness to variations. The Wright brothers designed a flying machine before the French even though they started experimenting at roughly the same time. The French 1) appealed to what little was known about the Wrights/Langley, 2) mental imaginings of what might succeed, and 3) guidance from growing flight experience. But “since [#1 and #3] were meager, however, the level of blindness, at least at first, was well nigh total.” What was the difference in the process between the Wrights and the French?