[spitx]

Agreed.

It should have read:

  "The arc of human progress is an illusory one" 

or

  "The arc of human progress - as defined by the narrow parameters of decreasing
   number of recorded human conflicts and genocides, declining number of
   incurable devastating medical conditions, improving/degenerating overall
   environmental health of the planet etc - is an illusory one." 

or

as you put it

  "true human progress is illusory" (whatever the parameters that determine it)

However what cannot be denied is that "liberal humanism", as Gray puts it, has come to wield the "pervasive power" it has now, in large part due to the advancements made by the West in the fields of science, technology and medicine and not despite of those advancements.

It's hard to make a case for universal "liberal humanism" when your own people are succumbing to famines in the millions.

Eg: The Great Famine in Ireland (1845–1852).

So simply put

  1. Knowledge is an absolute necessary element for the overcoming of 
     "cultural backwardness, blindness and folly" and to advance 
     "to ever more elevated stages of enlightenment and 
     civilization" and thereby the progress of humans.
  2. Human knowledge is said to be limited.
  3. Ergo, human progress will always be stunted by the said natural 
     limit of knowledge.    
  4. Further: There is no necessary condition that prevents humans from 
     reverting to the ways of the past once that limit has been reached.

[kijin]

> It's hard to make a case for universal "liberal humanism" when your own people are succumbing to famines in the millions.

I already said that I'm no fan of "universal liberal humanism". If someone thinks the Irish famine was progress because everything progresses all the time, they're wrong. But just because liberal humanism has problems doesn't mean that one must run to the other extreme. "If you hate my enemy, join my side" might be a useful tactic in war, but in philosophy people will just shrug and say "No way, you're both my enemies." My opinion is that humanity sometimes progress, sometimes stagnates, and sometimes regresses.

Anyway, here's my objection:

    1. Agreed.
    2. Agreed.
    3. Nope. Human progress will be limited by the aforementioned
       limit of human knowledge, but this is a very large limit,
       so there's plenty of room for progress before we hit the limit.
       We might have already hit the limit in some areas, but that
       doesn't mean we won't keep making progress in other areas.
    4. Agreed, but there is no necessary condition that says that
       humans MUST revert to an inferior state, either.
       Maybe they'll just stagnate until evolution produces
       a superior species with higher limits of knowledge. Why not?
       Just because X isn't necessary doesn't mean that
       not-X is necessary. Usually, they're both unnecessary.