Yeah, kind of undecidable, in similar way to nature vs. nurture debate. However, I would say that the case for scientific advancements being "in the air" and nearly inevitable is much stronger than, say, the inevitability of a paranoid egomaniac arising as a leader, rather than a more even-keeled and humane figure. Political action often seems balanced to go either way, for ill or for good, while scientific progress is in comparison mostly in a single direction, onward and upward.

I'd say that the case of scientific, as well as technological 'advancements being "in the air" and nearly inevitable' is much stronger than individuals effecting social changes. For example, Reformation happened not because of Martin Luther per se, but because of printing press that gave common people the Bible, and that spread the Luther's theses around Germany in a matter of weeks, and around the world in a matter of months (arguably, this was one of the first cases of pre-Internet outrage). If you were to remove Luther, a similar movement would likely start anyway, given that it was actually growing elsewhere in parallel! People we name and attribute changes to seem to be just puppets of the circumstances.

EDIT

Scott Alexander covers that one too:

http://slatestarcodex.com/2013/03/07/we-wrestle-not-with-fle...