We can reason about all kinds of things that don't exist in the brain...

For one You can’t actually prove this. We use brains to experience everything so we (humans as a species) can never know.

What does that even mean?

We can reason about fusion reactors, yet there's no such mechanism in our brains... How's that for a proof?

When we're reasoning about them, where are they?

Still outside. It's our thoughts about them, models, and concepts that are in the brain.

Plus, even if it was true, this wasn't the parent's point (that things exist in the brain while we're reasoning about them). His point (also wrong) was that different architectures must exist as structures in the brain (not as concepts we think and memories etc., but as parts of brains matter organization and wiring) for us to be able to reason about it.

> Otherwise we wouldn’t be able to reason about them

That's a bold claim.

I agree but not for the "otherwise" reasoning

I think it speaks to the complexity of the brain almost like every combination of numbers exists in pi

I disagree. No one built cars in 5000 BC but the confluence of ideas then have led to cars now.

This is a non-sequitur

On the contrary, I view it as a counterexample to "all architectures already exist in the brain for some use or another," which disproves your point. Let's not make the mistake of a fallacy fallacy here!

Perhaps you would like to expound or clarify your point to rule out the edge case of cars not existing in 5000 BC, but the models to derive cars 5000 years later suddenly came into being?