Why do the family members try to avoid talking of the ghost in "The Canterville Ghost"?

From their arrival at Canterville Chase, the Otis family are reluctant to talk about the ghost. In short, they do not believe in the supernatural because they are advocates of science and reason, as Mr Otis explains to Lord Canterville in Chapter One:

But there is no such thing, sir, as a ghost, and I guess the laws of Nature are not going to be suspended for the British aristocracy.

Similarly, when the family spends the day driving around the neighbourhood, they deliberately stop themselves from speaking about the ghost. According to the narrator, the family does this to prevent the "primary conditions of receptive expectation." In other words, if the family does not accept the possibility of a ghost, they are far less likely to experience any supernatural phenomena and this is crucial in safeguarding their scientific principles and beliefs.

This reluctance to talk about the ghost (so as not to accept his existence) reflects the culture clash which exists between the Otis family and the British aristocracy, one of the story's wider themes. The rational and scientific background of the Otis family acts like a barrier between these two opposing worlds and provides the story's conflict. (Please see the second reference link for more information on this theme).