biological naturalism: consciousness is a systemic property of the brain

this is a theory defended by philosopher John Searle.  (The Rediscovery of the Mind)

For Searle, consciousness emerges at certain levels of anatomical organization.

Certainly, the human brain with its approximate 100 Billion neurons (100x10⁹) and 125 Trillion synapses = 125x10¹² (just in the cerebral cortex alone!) has the complexity to generate consciousness.

This is probably true of the brains of nonhuman primates, which also have lots of neurons and neural connections. It is also true for other non human animals. It may not be true of snails, because they may not have enough neurons and interconnections to support (much) consciousness. It's not true of paramecia, because they don't have any neurons at all. And it's certainly not true of thermostats.

According to Searle, the mind (consciousness) is a systemic property of the brain. a biological phenomenon, a property of the brain, but not a purely functional property.

Systemic properties are very common in science, and some can seem quite unexpected just looking at the parts of the "system." For example, water is liquid, even though none of its parts, its molecules, are liquid. Liquidity is a systemic property. But we can explain why water is liquid in terms of its parts and their causal interactions. Another example is transparency – molecules aren't transparent; what makes glass transparent is the way the molecules are organized. In each of these cases, we can explain the "new" systemic property in terms of micro-level interactions.

Again, the mind is a systemic property of the brain. It is the brain as a whole that is conscious, even though its individual parts – neurons – aren't. Consciousness is caused by micro-level brain processes, and if the brain and its causal powers and processes were reproduced, so would consciousness be.

Consciousness cannot be eliminated from scientific discourse because objective, third-person descriptions of brain processes necessarily leave out the first-person subjectivity that lies at the core of phenomenal experience.  

First and foremost, consciousness entails first-person subjectivity. This cannot be reduced to brain-processes because any third-person description of brain-processes must necessarily leave out first-person subjectivity. For that reason, every attempt to reduce consciousness to something else must fail, because every reduction leaves out a defining property of the thing being reduced -- in this case, the first-person subjectivity of consciousness.