What does "seeks to build upon" mean?

Question

In Steven Pinker's Enlightenment Now there is a sentence S (below)

Harder to find is a positive vision that sees the world's problems against a background of progress that it seeks to build upon by solving those problems in their turn. (their refers to political movements and opponents which were previously noticed)

All examples of seek to build upon I found are of kind seek to build upon {something}, where {something} is idea, theory, etc., however in S I can't see any {something} (seeks to build upon by instead of expected seeks to build upon {something} by)

My explanation of seek to build upon {something} (based on sentences I found) is try to find something we could base or ideas or theories on. In S it unfolds to positive vision tries to be based on a progress by someone solving the worlds problems using this vision, which looks weird and unnatural

I mean, how can someone use vision if it is not yet based, however requiring someone to use it (chicken or egg problem)?

Answer 1

"Harder to find is a positive vision"

This is the basic sentence. It is inverted from the normal order "A positive vision is harder to find." for reasons of "end weight". The phrase "positive vision is modified by a long relative clause and it is convenient to place this at the end of the sentence. This implies that the basic sentence has to be inverted. This does not change the meaning.

The relative clause identifies the type of positive vision

a positive vision
    that sees the world problems against a background of progress...

And this progress is also described by a relative clause

progress
    that (the positive vision) seeks to build upon, by (...)

Thus the positive vision seeks to build upon (a kind of) progress

How does the positive vision build on this progress? It does so by solving those problems in turn.

That is the parsing and the meaning. The {something} is the progress discussed in the sentence.

However I confess, while I can parse this grammatically, I find the meaning hard to understand. Perhaps this because I am insufficiently familiar with the topic, perhaps it is just unclearly written.