Chapter 3 compares the UNIX philosophy and the design of UNIX with the philosophy and design of other operating system.

The first section describes the fundamental design ideas of the UNIX operating system, and develop the anti-UNIX, a system with the opposite of those ideas.
The anti-UNIX has no unifying idea (“just an incoherent pile of ad-hoc features”), has no internal boundaries to protect the user and the processes, has record structure and attributes for files and relies relies on binary file formats, has deficient scripting capabilities, and knows what is better for their users, and is non-hackeable.
In the end, I got the intuition that the author was describing MS-DOS as the anti-UNIX.

The chapter then describes others operating systems.
I know nothing or little about some of them.
That's the first time I heard about VMS, VM/CMS and MVS.
BeOS and OS/2 I know only a little about.

Reading this section I feel bad about BeOS; and pissed off about Windows NT.
BeOS seemed to be a good operating system, with the better of the UNIX and classic MacOS world.

The next section describes how UNIX survived from a server operating system into a client operating system, from big machines to PCs (and how the evolution in the opposite direction is more difficult to happen).

Chapter 4 is the first about the design of UNIX programs.

The chapter defends the design of encapsulated discrete modules with orthogonal APIs.
The modules should not be small in size and many in number, nor large in size and few in number. The first section talks about module density and how bugs likely distributes through the different module densities.
The chapter defends that modularization and their interfaces should be intuitive to the human mind.
It also defends that each module should be compact and have a “strong single center” (a single algorithm or general approach that defines the module).

I really like how the book cites other works through the text and the footnotes. I am collecting all the citation to check them later.