Is everything I think thought by my brain and am I nothing but my own brain?

Yes to your first question and no to your second.

Your brain is the vector for your thoughts to allow them to become expressed in the physical universe

But it's only the conduit or channel for your thoughts. Your thoughts are information which is non-physical and so are not limited by the same physical limitations as matter and energy are. It is just not accurate to say that your brain contains your thoughts. Yes, your brain expresses your thoughts but that is not the entire story.

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