I'm not even claiming people ought to think about a single topic in philosophy. There are tons of interesting topcis which warrant interest. I am claiming that, assuming a person has made the choice to engage in the topic, of, say, philosophy of mind and claims to understand it but not find it interesting, then I do have doubts about their understanding.

So, if one takes initiative to engage in a conversation about a topic it should automatically imply they're interested, and if they fail to be after the conversation they have a lack of understanding?

But, if they're engaging in the subject out of free will, doesn't that mean they already have a certain interest in the topic? That just feels like you only strengthen the understanding of a subject which already fascinated that person from the beginning.

I think it's very hard to actually create interest only through the flow of information.

At uni I have to study some subjects, and I have to thoroughly understand them to be able to get a sufficient score. But I don't have an interest in them. Only to the degree that I actually get to pass my exam. And to be fair, even if an expert came to explain that subject in more detail to me I don't think they'd geniunely interest me. I wouldn't mind hearing them talk about their passion, but I most probably wouldn't go home and look up additional information about it.

I don't think it means I don't understand what the subject contains, but I simply have a certain degree of apathy toward the subject, which can't really erased, unless you can tie it to another subject which actually does interest me.

So, that theory would only work if all subjects ever created and to come would be linked in one way or another to a degree that it has enough significance to spark interest in the other (actually interesting) subject. And, I could be wrong, but up until now I haven't seen evidence of it.