Regarding your last paragraph, I don't think I could do it. I can't think of a single time that I have thought of something I've never thought before where it hasn't been catalysed or at least shaped by external factors.

So to answer your question without that constraint, I've found good success in actively seeking out viewpoints that are different to mine. Some years ago, for example, I was a leftist who felt like ACAB was unreasonably inflammatory and "surely not all cops". I googled something like "Why ACAB is true", and found some compelling arguments. I tried to then debunk those arguments by trying to find evidence or reasoning against them, and I continued in that way for a while, and now I'm an anarchist.

It took a long while to get to the point of identifying as an anarchist, but the ACAB thing was a great example of what caused me to think in a way that was novel to me.

If we are talking about thoughts that are original not just with respect to me, but the wider world, I've found good success in reading outside of my field. I'm a scientist at heart, and I have studied biochemistry to a graduate level. That's the main field I work in. I'm a very stereotypical, systematic scientist, and that means I've not had much exposure to art or literature or other parts of the humanities.

I've been learning linguistics recently and there have been a few times that the way I have understood some core concept is distinctly different and perhaps even surprising to how my linguist friends would see it. Not wrong, just different. Everything I have learned is shaped by everything I've learned before, so I've found a lot of value in diving into fields that are outside of my wheelhouse.

Sometimes, I find concepts or methods that I don't understand well enough to say I know them (without more work), but even those are useful to me. They make me think in a way that is unfamiliar to my scientist brain, and that makes me more likely to have thoughts that no other scientist in the conversation has thought of before.

I've found a lot of utility and fulfillment in leaning into having a wide but not especially deep pool of knowledge surrounding my primary domain. I wish I could learn all of the things much more deeply, but I've come to appreciate the power of the paddling-depth water surrounding my main area.