The principles, much more the practice, need a good deal of scrutiny.
I didn't even see him, still less talk to him.
Much more (or
still more) is used when the grammatical form of the sentence is positive, and
much less (or
still less) when it is negative. Uncertainty arises when the form is positive but the sense is negative, as with adjectives in
un- and words like
difficult. In the sentence
It is difficult to establish all the facts, much less to reach a conclusion,
much more is strictly needed, not
much less, but the result is awkward and an alternative such as
let alone is often preferred.