We know that when the brain chemistry is disrupted, our consciousness is disrupted
You can test this yourself. Drink some alcohol and your consciousness will be disrupted. Similarly I am on Gabapentin for nerve pain, which works by inhibiting the electrical signals my nerves use to fire, and in turn makes me groggy.
While we don’t know exactly how consciousness works, we have a VERY good understanding of chemistry, which is to say, the strong and weak nuclear forces and electromagnetism (fundamental forces). Literally millions of repeatable experiments that have validated these forces exist and we understand the way they behave.
Drugs like Gabapentin and Alcohol interact with our brain using these forces.
If the interaction of these forces being disrupted disrupts our consciousness, it’s reasonable to conclude that our consciousness is built on top of, or is an emergent property of, these forces’ interactions.
If our consciousness is made up of these forces, then it cannot be a fundamental force as, by definition, fundamental forces must be the basic building blocks of physics and not derived from other forces.
There are no real assumptions here. It’s all a line of logical reasoning based on observations you can do yourself.