What is the "idea" that John says the narrator should not "let...enter your mind?

John doesn't want his wife to give into her natural creative imagination,

"I always fancy I see people walking in these numerous paths and arbors, but John has cautioned me not to give way to fancy in the least. He says that with my imaginative power and habit of story-making, a nervous weakness like mine is sure to lead to all manner of excited fancies, and that I ought to use my will and good sense to check the tendency. So I try."