Not much to say about this question. Both words are translated as "theory" in
English and I didn't find someone who explains the difference.

is there any difference?

Thanks, Or
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meaning
what's the difference between 理論 and 学説
Maybe I am not the most qualified person to answer this but I'll give my 2
cents anyway.

To me. 理論 seems to be much more common than 学説. But that's just based my
personal experience, so take it with a grain of salt. 理論 can be used as a
"prefix", so if you've been to Jisho.org, you'll notice there is an entry for
理論物理学 which means theoretical physics. 学説物理学 is absolutely nonsense.
Similarly, 理論化学 is theoretical chemistry​. Also 的 can be added after 理論 to
turn it into a Na-adj, i.e. 理論的. I pretty sure this is not possible for 学説,
but I could be wrong.
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I see it as something like this:

理論 ⇔ 応用 theory vs application

In contrast, 学説 would be an academic theory of some particular subject. 学説
also has a sense of _hypothesis_ that 理論 does not have.

So Tarou's モモンガ学説 would be Tarou's theory about flying squirrels -- say that
they are most active between 7pm and 8pm.

But 生物理論 would be biological theory.

Here's a
[知恵袋](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1458373844)
that confirms the idea I'm suggesting.
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