I suppose this could be interpreted as a philosophical question, but I'm asking it as an English one because I think there's a perfect word for this that's escaping me.
Some words I think are almost right:
Thought is the medium of intelligence.
Thought is the vehicle of intelligence.
Thought is a conduit for intelligence.
Maybe even: Thought is the stuff of intelligence.
I don't feel like any of these nail it though - can someone suggest a word that does? Or is one of the above the best I'm going to do?