Oil company representative: We spent more money on cleaning the otters affected by our recent oil spill than has been spent on any previous marine mammal rescue project.
Oil Rep's Position
The oil company spent more cleaning otters than any previous marine mammal rescue. This shows the company's concern for the environment.
Environmentalist's Conclusion
The oil company has no such (environmental) concern.
Environmentalist's Evidence
The company admitted that photos of oil-covered otters would be particularly damaging to its public image, which affects sales. So the company's real concern is sales.
Evaluate
The environmentalist leaps from "real concern is sales" to "no concern for the environment." That works only if the company can't have both concerns at once. If a company can have multiple motives — sales-driven and environment-driven — then evidence of a sales motive doesn't establish absence of an environmental concern.
Goal
Find a sufficient assumption: if it's true, the conclusion follows. The environmentalist needs the principle that the company can't have more than one motive for cleaning the otters.