To even begin to decide whether this claim is scientific (testable), thoughts and their content need to be defined physically here.
And why care about pre-given thoughts? I care about my emotions more. Thinking is just an utility guided by them. "What you think is good" is certainly more about emotion than ideas. One can walk back from their emotion, starting from a clean sheet, gradually reconstructing the desired idea by bit - most languages have the necessary primitives available.
> The idea of purely independent thought is an illusion.
There's no need for one's thinking to be dependent on anything more or less than reality relevant to one. No culture other than one's private own needs to be included as dependency. No need to copy or almost copy another culture as well.
> It follows that independent action is also an illusion (your idea of what to do next, as well, tends to be borrowed!)
My idea of what to do next comes from my rationality. Rationality can be rebuilt independently, at least in part that covers a big part of everyday life, when a minimal amount of education consisting of self-evident truths is given. So my behavior is only dependent on me having acquired a number of ideas useful to me individually that I have also sanitized.
> 1. All thought is fantasy
"All even integers are divisible by 2." What are you trying to imply? Fantasy can be made to conform to rules and to be useful directly as functional data.
> 3. Most thought is borrowed
"Most" by what metric?
> 5. Society is the manager of thought, not the individual
I think that an individual can become the "manager" of their own thinking as well by degree such as allowed by their cultural environment plus that of their capability.
> 6. Thought advances as a whole
What?
> 7. Without collective thought as a substrate, the individual won't function
Please explain in more detail why and what is 'collective thought'.