Where should we draw the line between public and private?

In a previous question about privacy, I asked why people wanted privacy. A number of people said, “because it is body’s business.” To me, that’s kind of a non answer.

It seems to me that saying that some activity is nobody’s business begs the question. Why isn’t that activity anyone else’s business? Is the presumption that something is private until it becomes someone else’s business? Then at what point does a formerly private activity become someone else’s business?

Why do we presume privacy at all? We might as well presume everything is public until it falls below the level of interest of the public. Where is that point?

I suspect that there will be many different lines between public and private that people draw. People may draw different lines for different activities. If you offer examples of what you are talking about, it would help.

If you want some guidance, consider these situations:

If an undiagnosed sociopath is planning torture and murder and is collecting the instruments he needs in his basement, is that the public’s business?

If a farmer is storing pigshit on his farm next to a stream, is that the public’s business?

If a person is storing legal weapons and legal bomb making materials in his garage, is that the public’s business?

If a mentally challenged woman is having sex with no birth control, is that the public’s business?

If a parent is spanking a child, is that the public’s business? What about if the parent is not touching the child physically, but is constantly demeaning the child?

If a company is spying on another company, is that the public’s business (remember, companies are treated as individuals in the US).

If a company is using very complex financial mechanisms, is that the public’s business?

If a politician is cheating on his wife, is that the public’s business? What about if your neighbor does the same thing?

For all of these things, tell us why it is or is not the public’s business.