While science fiction abounds with intelligent trees and fantasy is replete with such beings as dryads, the subject of trees thinking has generally been something used to mock the absurd pretentious of philosophers. However, this is starting to change: scientists are starting to seriously consider that trees have mental states, such as feelings. This lends the discussion of thinking trees a cloak of credibility that I will now avail myself of.
From a philosophical standpoint, the key issue is whether a tree can have a mind. Unfortunately, philosophers do not agree on the nature of the mind. As such, I will consider some of the main theories of mind and whether they would allow for thinking trees.
There are a variety of philosophic theories which attempt to explain the mind. Some of the better known ones include identity theory, substance dualism, property dualism and functionalism. The implications of each will be considered in turn.
Identity theory is a materialist theory of mind—the mind is composed of matter. Identity theorists assert each mental state is identical to a state of the central nervous system. So, the mind is the central nervous system and its states. Given identity theory, trees cannot think. This is because they lack a central nervous system.
Substance dualists claim that reality contains two fundamental types of entities: material entities and immaterial entities. On this view, which was embraced by Descartes, the mind is an immaterial substance which has a causal relation with its body. This mysterious relation enables the mind to control and receive information from the body and allows the body to affect the mind. On this view, a tree could have a mind—a tree having an incorporeal mind connected to its material shell is no more mysterious that a human having an incorporeal mind. It is also no less mysterious.
A second type of dualism is property dualism: the mind and body are not distinct substances. Instead, the mind is made up of mental properties that are not identical with physical properties. For example, the property of being the feeling of sadness could not be reduced to a physical property of the brain, such as the firing of certain neurons. So, the mind and body are distinct, but not different substances.
This sort of dualism would also allow for trees to think. This is because the theory does not require the physical properties to be composed of the same properties that make up the human nervous system—all that would be needed is the right sort of mental and physical properties. Again, that trees could have this metaphysical makeup is no stranger than the belief that humans do.
The last view to be considered is functionalism. There are many varieties of functionalism, but they all have a common foundation: mental states are defined in functional terms. A functional definition of a mental state defines that mental state in terms of its role/function in a mental system of inputs and outputs. To illustrate, a mental state, such as being in pain, is defined in terms of the causal relations it has to external influences on the body, other mental states, and bodily behavior.
Functionalism is usually taken to be a materialist view of the mind because the systems are supposed to be physical systems. While identity theory and functionalism are both materialist theories, they differ in a critical way. For identity theorists, each mental state, such as being sad, is identical to a physical state, such as the state of neurons in a chunk of the brain. For two mental states to be the same, the physical states must be identical. So, if mental states are states of neurons in a certain part of the human nervous system then anything that lacks this sort of biological nervous cannot have a mind.
The functionalist has a rather different view: a mental state, such as feeling sad, is not defined in terms of a physical state. Instead, while functionalists believe each mental state is a physical state, for two mental states to be the same they need only be functionally identical. So, if mental states are defined functionally, then anything that is capable of exhibiting these functions can have a mind. While trees obviously lack the brain and nervous system of a human, they could have physical systems that function in analogous ways. To use an obviously analogy, different computer hardware can run the same programs. For example, this essay can be read using a browser on a wide variety of hardware platforms including an Android phone, an Xbox One and an old Macintosh with a Motorola chip.
While the issue of whether trees do think or not remains, this essay has addressed the issue of whether they could have minds within the context of modern philosophy of mind. If dualism, property dualism or functionalism is correct, then trees could have minds and think. However, if identity theory is correct, then trees cannot think.