I didn't acutally /quite/ say that "but" == "and" in English. I said that they have the same "denotations", but different "connotations". A word's denotation is it's plain meaning -- what it says on it's face. Quite literaly, what it denotes. That's the meaning that programming languages can hope to capture. That's the meaning of words that scientific discourse and legalisims should revolve about. It's simple, clear, and to the point. "connotations" are the more elusive meanings of worlds. They're what makes poetry and prose interesting. They're shades of meaning, hints at something beyond the mear denotations. They're why two words that are synonyms can give much different effects when said. They're what makes translation really difficult.

Another concrete example... saying to a woman that "danm, your tits are hot" fits in a different conversation then saying to her "I find your breasts highly attractive". (I say, not being a woman, and being the sort to use the first phrase.) The words have the same denotations. They have very different connotations.

If you interchange "but" and "and" in a sentance, the sentance still has the same meaning, but different implies something different.