What do "the more" and "the less" modify in "The more our knowledge of things is certain and particular, the less it is possible for us to feign"?

The following is an excerpt from The Young Spinoza: A Metaphysician in the Making.

(1) The more our knowledge of things is certain and particular, the less it is possible for us to feign;

What do the more and the less modify in this sentence?

I thought the more modified is and the less modified feign. But

• (2) As our knowledge of things is certain and particular more, so it is possible for us to feign less.

sounds strange.

Is it possible that the more modifies certain and particular and the less modifies possible?

Then, the sentence can be paraphrased as

• (3) The more certain and particular our knowledge is, the less possible it is for us to feign.

But, (3) is not the same as (1) grammatically. Or is it?

What we have here is not about more or less modifying anything.
Rather, this is an example of an idomatic syntactic construction, often called

• the X-er, the Y-er

It does not form a normal sentence, but rather connects two phrases containing comparatives, and indicating that there is a direct correlation between changes in the first phrase and the second.
An inverse correlation is easily done by changing the valence of the comparative (less instead of more), or of the degree predicate being compared (uglier instead of prettier).

Either or both of the phrases may contain clauses, but don't need to.
There are many fixed phrases, proverbs, and common usages.
Examples:

• The older I get, the more I enjoy life
• The bigger, the better (fixed phrase)
• The bigger they come, the harder they fall (proverb)
• The more carefully I consider his proposal, the less I am convinced that it's a good idea.

In each case, a delta-x in the first phrase leads to a delta-y in the second.
What x and y indicate is the degree of whatever is compared in either phrase --
in particular, the examples above indicate that

• how old I am correlates directly with how much I enjoy life
• how big (something is) correlates directly with how good (it is)
• how big (an opponent) comes (at one) correlates directly with how hard they fall
• how carefully I consider the proposal correlates inversely with my opinion of it