"[On Earth], man had always assumed that he was more intelligent than dolphins because he had achieved so much — the wheel, New York, wars, and so on — while all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man — for precisely the same reasons."
This is for when two sides share a particular assumption, but have opposite ideas of what it implies because of opposite opinions.
Page quote shared with Sapient Cetaceans
; if a suitable alternative can be found (I'm sure I've seen this elsewhere) it can be replaced, and if not, there's other potential page quotes for Sapient Cetaceans
EDIT: See this
thread for clarification of distinction between assumptions and opinions.