Kiwi, I quite agree with your first paragraph, except possibley the conclusion. Surely, as you say, one's thoughts are derivative of one's experience. But if that means that every idea is "by definition derivative" -- that is, that there is no such thing as an "original idea" (that the last phrase is an oxymoron) -- then how did any ideas EVER arise?
And quite clearly they do arise from time to time -- else no ideas would ever enter the human experience, and we would have no more ideas than did our caveman ancestors.
The distinction is, perhaps, that an idea is created by adding one's insight to the underlying experience. That addition is a creative act, much as baking a cake is to create something, even though that cake is "derivative" of the flour, milk and eggs. In other words, an idea (like a cake) can be original, though it will of course derive from and extend pre-existing ideas and knowledge.
Of course, what I just said does not address tsuwm's challenging question of how often, and by what process, original ideas arise.
