Is the book Genki wrong about volitional + と思っています?

Question

I've been looking for this on the web but can't find my specific doubt.

I just read the Genki 2 chapter about volitional + と思っています to say you are planning on doing something. The book is pretty clear that this grammar can be used ONLY to express our own opinions or intentions and not other's.

But right after the lesson, the first exercise says: "Describe what each person is planning to do" and the example follows:

メアリーさんは日本でいろいろな経験をしようと思っています。

The book just said is used for our own intentions but then the example is trying to tell us that another person is intending to do as if we know other person intention.

So, can this grammar be used to express other people's intention? Thank you beforehand.

よろしくおねがいします。

Answer 1

A Dictionary of Basic Japanese Grammar (Seiichi Makinko and Michio Tsutsui) has this to say on the subject:

When the subject is not the first person, the nonpast form of 思う cannot be used. ... The reason why this is unacceptable is that 思う represents an internal feeling of the speaker alone. Therefore, when the subject is the third person, 思う has to be replaced by the stative 思っている which means ' he (= the third person subject) has indicated that he feels ~, in such a way that the speaker can see and/or hear what he feels'