"Is lean a bona fide management science based profession or a tool based craft? I'll suggest that current practice and teaching is more the latter than the former and because of that, the influence of lean is far inferior to its potential."
Within Manufacturing, management, engineering, and even consulting are professions. "Lean" per se is not a profession, but a loosely defined body of knowledge that all manufacturing professionals should possess to some extent.
Like Spear, we all tend to think of mechanical engineering as an application of Newtonian mechanics. In reality, however, it is not as if the field had developed from scratch based on Newton's theories.
People had been making mechanical devices long before, and mechanical engineering as we know it actually came from the grafting of Newtonian mechanics onto an existing body of craft-based, empirical know-how.
As Takahiro Fujimoto pointed out, the Toyota Production System (TPS) was never designed from first principles but instead emerged from the point solutions and countermeasures Toyota employees came up with to overcome a succession of crises in the development of the company. What is remarkable is that they did coalesce into a system.
Lean is supposed to be a generalization of TPS to contexts other than car manufacturing at Toyota. The challenge of developing Lean is to reverse engineer principles from tools.
Over the past 35 years, many Japanese publications have described TPS, with authors like Taiichi Ohno, Shigeo Shingo, Yasuhiro Monden, Kenichi Sekine, Takahiro Fujimoto or Mikiharu Aoki...
These publications have made many of the tools of TPS accessible to anyone willing to study them. They have been less effective, however, at showing how the tools work together as a system, and even less at spelling out underlying principles. It is something I have attempted in my books.
Little of the content of TPS has made its way into Lean, as promoted and practiced in the US and Europe, where it boils down to drawing Value-Stream Maps and running Kaizen events that have little to do with TPS.
TPS still needs to be studied, and its essence abstracted into a theory that is neither false nor trivial and provides principles that can be practically deployed as needed in new industries. I agree with Spear that there is great value in such a theory, but it has to exist before we can use it.