The relevant quotes are:
"The nonhuman remains nonhuman. He can only show us those facets of himself which we can understand. Thus he often seems to be a two-dimensional, even comic personality. But remember, we have the corresponding effect on him."
-Poul Anderson, The Van Rijn Method (New York, 2009), pp. 264-265.
"'[Nonhumans] are too unlike us. You probably know better than I how vastly their psychologies, instincts, drives, capabilities differ from ours, and from each other's...I think we interact with them, and they with us, only on a rather superficial level. Partnership is possible between human and alien, yes. Sometimes even what the human feels as friendship. But how does the alien feel it? That may be ultimately unknowable, on either side.'"
-Poul Anderson, For Love And Glory (New York, 2003), Chapter IX, p. 53.
Bob told me that he never had conversations between human beings and aliens but there was some conversation with hyper-dimensional beings in the Orbitsville Trilogy and maybe between souls of different species in the scientifically rationalized hereafter of The Palace Of Eternity? In old age, we try to remember works read literally decades ago.
Sagan suggests a means of communication. See here.
Niven and Pournelle
When, in Anderson's A Circus Of Hells, an alien on her own planet shakes her head in disbelief, we ask, "Would she do this?" whereas, when one of Niven's and Pournelle's Moties nods her head, we soon realize that this is because she is trying to converse with human beings and is learning fast.
I am definitely signing off till next month. Excelsior.