> The marginal argument is confusing to me.

When economists say “marginal” they usually mean what an engineer would call “derivative”. So “marginal cost”, for example, is usually “d(cost)/d(production)” or “d(cost)/d(sales)”. Similarly, marginal productivity means “d(productivity)/d(workers)”.

Usually this pops up in ideal economics because under ideal circumstances, maximizing revenue and productivity and so on means “set the derivative of something to zero” to find the optimum point.

(Disclaimer: I’m a physicist not an economist, but I’ve taken an intro economics course. The above was my main takeaway from that…)

That's very insightful. Thank you.