Why is the topic marker often used in negative statements (ではない, ～とは思わない)?

Question

What function does は provide in statements such as 本ではない or 本だとは思わない? I notice the は after と is often left out, at least if a Google search for 思わない is any indication, but there still seems to be a general trend of sticking は into negatives where I presume they wouldn't be used in positive sentences (one hears です and である, but not ではある). What's going on here?

Answer 1

The topic particle WA is normally used with negative statements. As WA is used to highlight the specific thing under discussion, you can think of it as "not the case for X, but the case for something else."

Terebi ha mimasen - I don't watch TV (but I watch movies).

WA is also usually dropped if something is already filling in for the WA clause (kono hon wa taisetu de aru). WA may also be dropped stylistically in some cases, or if another particle (GA, MO) are filling in, or if teh topic of conversation is already understood.