1. Psychology ↑
Studies of the mind branched from philosophy to psychology in the latter half of the 19th century. By 1874, the year of Irish natural philosopher John Tyndall's Belfast address, French physician Pierre Paul Broca and German neurologist Carl Wernicke had identified areas of the brain associated with different aspects of language.
In 1879, German physician Wilhelm Wundt founded the first laboratory dedicated exclusively to psychology, and German psychologist Hermann Ebbinghaus became the first person to describe the 'learning curve' during his studies on memory in 1885. Ebbinghaus is also known for discovering the Ebbinghaus illusion, an optical illusion that illustrates our relative perception of size.
The Ebbinghaus illusion. Image credit: Ebbinghaus/Public domain.
2. The Neuron Doctrine ↑
In the 1870s-1890s, Italian neuroscientist Camillo Golgi and Spanish neuroscientist Santiago Ramón y Cajal developed tissue-staining techniques that allowed them to map the path of nerve cells, known as neurons, in the brain.
Neurons are cells that can gather and transmit information via electrical signals, along a neural pathway. We now know that there are three types of neurons: sensory neurons, motor neurons, and interneurons.
Sensory neurons carry signals from the outer parts of the body to the central nervous system, and motor neurons do the opposite. Interneurons carry information between motor and sensory neurons.
There are about 100 billion neurons in the human brain, and the amount of electronic signals they emit depends on how much stimulus they are receiving. Neurons emit more electronic signals per second, and therefore have a higher electronic frequency, when they receive more stimuli.
The discovery of neurons led to the development of the neuron doctrine, first proposed by German anatomist Heinrich Wilhelm Gottfried von Waldeyer-Hartz in 1891. The neuron doctrine states that neurons are the fundamental units of the nervous system and are responsible for all brain activity.
3. Psychoanalysis and Behaviourism ↑
In 1913, American psychologist John Watson suggested that psychology should be a science of behaviour, not of the mind. Behaviourism rejects the idea that internal mental states like qualia exist, and arouse in the early 20th century as a response to Austrian psychologist Sigmund Freud's psychoanalysis.
Watson was influenced by the idea of classical conditioning, discovered by Russian physiologist Ivan Pavlov in 1897. In 1920, Watson and his assistant Rosalie Rayner used classical conditioning to make an eleven-month-old baby, Albert, fear stimulus that would not normally be feared, such as a white rat. Albert soon generalised the response so that he was afraid of anything furry.
Further criticisms of psychoanalysis arose in 1948, when American psychologist Burrhus Frederic "B. F." Skinner argued that thoughts and feelings are merely examples of behaviour.
Three years later, German psychologist Hans Eysenck showed that some personality traits are genetic and, in 1963, Austrian-British philosopher Karl Popper claimed that psychoanalysis is not a science because it cannot be falsified.
Popper argued that science can be distinguished by the fact that it makes unique predictions that can be proven false.
4. Identity Theory and Functionalism ↑
In 1959, American linguist Noam Chomsky showed that behaviourism cannot explain how people can produce an infinite variety of sentences, unique in both structure and meaning. He suggested that there must be internal mental structures, and behaviourism was soon replaced with identity theory.
Identity theory suggests that particular states of the mind are identical to particular states of the brain. Identity theory was first suggested by British psychologist Ullin Place in 1956, and was extended by Australian philosopher Jack Smart three years later. This approach was criticised by American philosopher Hilary Putnam in 1960.
Putnam showed that the same sensations can arise from different brain states, an animal may see the same thing as a human, for example, but this image will correspond to different parts of their brain. Putnam suggested that the mind is more like a computer.
In 1965, American philosopher Jerry Fodor suggested that the mind can be explained in functional terms, this means anything that performs the function of a mind, is a mind, and must therefore be conscious.
5. Cognitive Science ↑
British chemist Christopher Longuet-Higgins coined the term 'cognitive science' in 1973. Cognitive science accepts the neuron doctrine. It shows that signals in the brain are mostly sent through networks composed of millions of neurons.
Neurons receive stimulus from branches known as dendrites and then communicate this information across a synapse, either electrically or chemically.
At electrical synapses, a nerve impulse is fired along an axon, this crosses a gap junction to connect to the dendrite of another neuron.
At chemical synapses, electrical impulses are converted into chemical signals. This process is called exocytosis and it is slower than electrical transmission.
Complete neuron cell diagram. Image credit: LadyofHats/Public domain.
All material theories of the mind must be able to solve the problems raised by French natural philosopher Rene Descartes in the 1600s. They must be able to explain the existence of qualia, state where qualia exist within the brain, and explain why it does not appear possible to divide or duplicate our consciousness. Material theories of the mind must also be able to explain the inherent subjectivity associated with our conscious experiences.