Doris: I've noticed that everyone involved in student government is outspoken.
Doris's 1st Claim
Everyone in student government is outspoken. Does Zach say some members of student government aren't outspoken?
No he explicitly agrees with this claim.
Doris's 2nd Claim
If we want students to be more outspoken, we should encourage them to become involved in student government. Does Zach object to the idea of encouraging people to get involved in student government, as a way to get students to be more outspoken?
Yes, kind of! This conditional form that Doris used, "If you want A, you should do B" implies a causal relationship: B is a way to cause more A to happen.
Doris was thinking that getting a student to be involved in student government is a way to get students to be more outspoken. She's assuming that the reason she's noticing that everyone in student government is outspoken is because being involved in student government makes you more outspoken.
Zack is countering this assumption by offering an alternate explanation for that correlation: the outspoken nature comes first, and that's what gets them into student government.
Since he sees the causality going the other way (outspoken ⇢ involved in govt), he says that Doris's plan to get people to be more outspoken by having them join student government is not gonna work.
Goal
They disagree explicitly about this plan. Doris thinks that getting people involved in student government could make them more outspoken. Zack doesn't think that will work.
But they also disagree implicitly in terms of their causal assumptions about why there's such a strong correlation between being in student government and being outspoken.
Doris thinks it's because being in student government makes you more outspoken. Zack thinks it's because being outspoken makes you want to get into student government.