Sales manager: The highest priority should be given to the needs of the sales department, because without successful sales the company as a whole would fail.
Sales manager's argument
The sales department should get the highest priority because successful sales are necessary for the company's success.
Shipping manager's response
Several other departments are also necessary for the company to succeed. But you can't give the highest priority to all of them — that's impossible.
Evaluate
The sales manager's implicit principle: if a department is necessary to the company's success, it deserves the highest priority. The shipping manager doesn't deny that sales is important — instead, the response shows that sales manager's principle, applied consistently, would absurdly require giving "highest priority" to several departments at once. By definition, only one thing can be the "highest" priority — so the principle leads to an impossible conclusion.
Goal
Find an answer that describes the response as exposing an absurd consequence of the sales manager's underlying principle.