In the early 20^th century, psychology is dominated by the unscientific methods mesmerism (1), phrenology (2) and spiritualism (3). Psychologists start to explicitly oppose pseudo-science and the scientificity of psychology becomes a theme.

Positivism states that science is the way to the truth and, therefore, the objectivity of knowledge must be guaranteed. Eugenics refers to a social philosophy which claims that the fate of a nation can be improved by selective breeding of the inhabitants. Intellectualism refers to obtaining knowledge for the sake of knowledge.

Einstein demonstrates that part of Newton’s theory was false. In a reaction to this, Bridgman introduces operationalization in physics. However, there are two objections to the use of operationalization in natural sciences:

• The same concept can be measured in two different ways (e.g. length).
• The operations cannot be synonymous to concepts.

Behaviourism was influenced by positivism (1), eugenics (2), a mistrust of intellectualism (3) and operationalization (4). It led to comparative psychology, the study of behaviour of animals with the intention to shed light on human functioning within the framework of the evolutionary theory. Behaviourism studies the relationships between stimuli and behaviour.

The behaviourist ideals of science are using mathematical laws (1), using operationalization (2), using independent and dependent variables (3) and striving for verification (4).

Watson has several important characteristics:

• Proposes a psychology without mental terms, solely based on stimulus-response sequences (1), reinforcement (2) and punishment (3).
• Uses the idea of tabula rasa in his proposal for behaviourism.
• Defines psychology as an objective experimental branch based on prediction and control of behaviour.
• Designs behaviourism as an absolutist system.

Pavlov invents the first learning model using a behaviourist approach and uses classical conditioning on dogs. He believed psychology could be reduced to physiology and that thinking consisted of reflexes. Thorndike formulates the law of effect – behaviour followed by a reward is more likely to be repeated – and discovers this through animal research using Thorndike’s puzzle-box. By doing this, Thorndike demonstrates operant conditioning. Pavlov and Thorndike’s research leads to the notion that human behaviour is equal to an animal’s behaviour.

Skinner has several important characteristics:

• Proposes radical behaviourism; a total ban on the use of terms that refer to mental states.
• Proposes that all behaviour can be understood in terms of stimulus-response relationships; he proposes that everything that is not directly measurable should be left out.
• Proposes that man is the point where all influences come together, meaning that free will does not exist.
• Proposes that there is no body-mind problem as there is only input and output.

Tolman founded purposive behaviourism – behaviourism which sees behaviour as goal-related – through doubting Skinner’s radical behaviourism.

Behaviourism attempts to explain all behaviour, although this is not possible. For example, in Skinner’s theory of language, a child starts with mimicry and learns language through reinforcement and punishment. However, this is not the case.

Chomsky opposes Skinner’s radical behaviourism and his theory on language and has three counter-arguments against Skinner’s theory:

1. Skinner’s theory is just as vague as traditional theories
   The terms in Skinner’s theory are supposed to be more objective because the terms (e.g. stimulus) refer to observable things and behaviour. However, these terms do not apply to everything when taken literally and when the terms are taken metaphorically, the terms are vague.
2. Skinner’s theory cannot explain the complexity of language
   Language requires intention, whereas Skinner’s theory is limited to stimulus-response sequences.
3. Skinner’s theory does not do justice to how children learn language
   Children do not learn language through reinforcement and punishment as parents are not precise enough in reinforcement and punishment. Children do also not use mimicry in language acquisition and the mistakes children make does not suggest trial and error learning, which is the case, according to Skinner’s theory.

Skinner attempts to establish a lawful relationship between stimulus and response. However, this is not always possible. For example, if the stimulus is a red chair, then the response can differ between people (e.g. ‘red’, ‘chair’). The responses can differ for one stimulus, making the stimulus a personal property, rather than an objective stimulus

According to Chomsky, reinforcement is also too vague in a natural situation as it is impossible to pinpoint what the exact reinforcement is. In a natural situation, everything could potentially be a reinforcement.        

Chomsky states that language capability requires mastery of grammar, although this cannot be learned inductively with stimulus-response associations. Therefore, grammar is a theory that cannot be derived from the data available to a child, meaning that a child must have innate knowledge of grammar. This is called the poverty of the stimulus argument.

Chomsky has several important characteristics:

• Opposes Skinner and his radical behaviourism
• Nativist; there is innate knowledge in the form of a language acquisition device (LAD).
• Proposes that linguistics should view the meaning of a word as a mental concept rather than view the meaning of the world as being in the observable world.
• Proposes that a system (e.g. LAD) should not be omitted from an explanation because it is complex.

The language acquisition device is controversial as it cannot be localized in the brain (1), it is possible to question how poor stimuli really are (2) and because it appears that aspects of grammar can be learned inductively (3).

Turing has several important characteristics:

• Demonstrates how to make a machine do calculations
• Proves that a machine can handle all calculable functions, including standard logic (e.g. ‘p and q’).
• Demonstrates how a psychological process (e.g. reasoning) can be performed by a machine.

Machines started using Boolean algebra which has a couple of rules:

• True sentences are given the value of ‘1’.
• False sentences are given the value of ‘0’.
• The truth of compositions (e.g. ‘p and q’) is a function of the truth of the elementary statements (i.e. ‘p * q’).

Turing’s insights demonstrate that the mind could be implemented in the brain, giving a new twist to the dualism/materialism debate. It leads to the analogy of the brain as a machine and the mind as software. This analogy allows for the cognitive revolution, viewing humans as information processing systems.

The availability of computers changed research in psychology in three ways:

1. New purposiveness of behaviour
   The availability of computers demonstrated that goal-oriented behaviour could be explained through information feedback.
2. Simulation of human thinking
   The availability of computers allowed psychologists to simulate the hypothesised psychological processes using computer programmes.
3. Clarifying the role of the psychologist
   The availability of computers and the accompanying analogy allowed psychologists to clarify their role regarding the study of the mind (i.e. ‘software programmers’).

Information feedback refers to a mechanism in which the current performance level is compared to the desired end-state and the discrepancy is used to bring the performance closer to the end-state aimed for. Information processing refers to encoding mental representations, transforming them by means of algorithms and integrating them with existing knowledge.

Cognitive psychology introduced top-down processes, which refers to the fact that information from higher-processing stages is fed back to previous processing stages and influences processing at these stages. The models designed by cognitive psychology lead to predictions which could be tested in experimental settings.

Scientists have tried to convince society that scientific knowledge is superior to humanist knowledge. The reasons are that science is based on observation and experimentation, leading to true findings (1), scientific theories are summaries of empirical findings and are, therefore, true (2) and scientific knowledge is infallible and should be the motor of progress (3).