He says that “all of our mental phenomena are caused by lower level neuronal processes in the brain and are themselves realized in the brain as higher level, or system, features. The form of causation is “bottom up,” whereby the behavior of lower level elements, presumably neurons and synapses, causes the higher level or system features of consciousness and intentionality.”
As this view emphasizes the biological character of the mental, and treats mental phenomena as ordinary parts of nature, he calls it “biological naturalism.” In essence biological naturalism is a middle position between materialism and property dualism.
Here’s how Searle explains his objections to property dualism:
"The property dualist and I are in agreement that consciousness is ontologically irreducible. The key points of disagreement are that I insist that from everything we know about the brain, consciousness is causally reducible to brain processes; and for that reason I deny that the ontological irreducibility of consciousness implies that consciousness is something “over and above”, something distinct from, its neurobiological base. No, causally speaking, there is nothing there, except the neurobiology, which has a higher level feature of consciousness. In a similar way there is nothing in the car engine except molecules, which have such higher level features as the solidity of the cylinder block, the shape of the piston, the firing of the spark plug, etc. 'Consciousness' does not name a distinct, separate phenomenon, something over and above its neurobiological base, rather it names a state that the neurobiological system can be in. Just as the shape of the piston and the solidity of the cylinder block are not something over and above the molecular phenomena, but are rather states of the system of molecules, so the consciousness of the brain is not something over and above the neuronal phenomena, but rather a state that the neuronal system is in."Further, he says:
"I say consciousness is a feature of the brain. The property dualist says consciousness is a feature of the brain. This creates the illusion that we are saying the same thing. But we are not… The property dualist means that in addition to all the neurobiological features of the brain, there is an extra, distinct, non physical feature of the brain; whereas I mean that consciousness is a state the brain can be in, in the way that liquidity and solidity are states that water can be in."Edward Feser has written a paper, “Why Searle Is a Property Dualist,” in which he argues that Searle’s anti-materialist arguments in philosophy of mind entail property dualism.
According to Feser, property dualism is unavoidable in the way in which Searle describes his theory of biological naturalism. “If the physical processes which cause consciousness are objective third-person phenomena, and consciousness and other mental phenomena are subjective or first-person in nature, it is reasonable to describe the latter as being of a fundamentally different kind than the former. That is, it is reasonable to say that there exists in the universe a dualism of properties,” Feser writes.
Here’s another interesting excerpt from Feser’s paper:
“If paradigmatically and uncontroversially physical phenomena are essentially objective, and paradigmatically and uncontroversially mental phenomena are irreducibly subjective, then it follows that they are of fundamentally different metaphysical kinds. It follows, that is, that property dualism – the claim that there are (at least) two metaphysically fundamental kinds of property in the universe – is true. Since Searle accepts the antecedent, he is committed also to the consequent, whether he realizes it or not and whether he wants to refer to that consequent by its usual label 'property dualism,' or instead by the label 'biological naturalism.'”In my view, Feser offers very convincing arguments to prove that some kind of property dualism is there in John Searle’s thinking. I am convinced by Feser’s arguments.