Descartes has several important characteristics:

• Introduces the mind-body problem through Cartesian doubt.
• Introduces the interaction problem.
• Dualist; he proposes that the mind is immaterial and the rest is material.
• Nativist; he attempts to prove the existence of God and uses innate knowledge for this.
• Mechanistic thinker; he believes the body can be seen as a machine.
• Rationalist; he believes that the only way to obtain knowledge is through the ratio.

The mind-body problem refers to the problem of the relationship between the (material) body and the (immaterial) mind. The interaction problem refers to how an immaterial substance can interact with a material substance.

Cartesian doubt refers to systematically doubting all previous beliefs and reasoning back from what remains. Descartes did this by putting all beliefs on an imaginary table and getting rid of all fallible things. Observation is fallible and, therefore, is not certain. Reality is fallible as it is possible that everything is fed to us by an evil demon (i.e. simulation) and, therefore, is not certain. There is something which doubts and this is not fallible and is, thus, certain (i.e. “Cogito Ergo Sum / I think, therefore, I am”).

Descartes states that there is an idea of perfection (i.e. Plato’s ideal world) and this idea cannot be his own as he is not perfect, meaning that this idea has to be planted in him by a perfect being: God. This idea of perfection is innate.

Descartes’ dualism is opposed by modern physics as it violates the law of conservation; adding energy out of nothing. Rejection of dualism easily leads to materialism but does not necessarily lead to reductionism as holism is also a viable alternative. Reductionism is the view that theories of mental properties can be reduced to neuroscientific theories.

Locke has several important characteristics:

• Attempts to refute rationalism using psychological observations; the observation of children
• Empiricist; humans are born as a tabula rasa and all knowledge comes from experience.

The idea of the tabula rasa has political implications as this implies that at birth everyone is the same, giving rise to natural rights, a predecessor of human rights. The idea of innate differences is often viewed as a threat to human rights.

It was believed that everything was nurture until well in the 20^th century. This belief was changed by research into genetics which demonstrated that predispositions exist (1) and that different psychological properties were influenced by genes. Predispositions can be seen as a form of innate knowledge.

Berkeley has several important characteristics:

• Empiricist; all knowledge comes through the senses.
• Idealist; people can only be certain of their perceptions rather than a material external world.
• Proposes that there is no material external world.
• Proposes that there are only spirits and minds that perceive.
• Proposes that God causes the ideas in the mind, therefore, creating continuity in perception.
• Proposes that all properties of reality depend on the mind (i.e. “esse est percipii / to be is to be perceived”).

According to Berkeley, the world can only be perceived indirectly. A material world does not cause ideas. The ideas themselves also do not cause ideas (read: perception) as the ideas are passive. The mind also does not cause ideas, as the mind cannot choose which ideas to observe. The ideas are caused by another spirit, which is superior (i.e. God).

Idealism refers to the view that human knowledge is a construction of the mind (i.e. for Berkeley: God’s mind) and does not necessarily correspond to an outside world.

Hume has several important characteristics:

• Introduces the problem of induction
• Empiricist: experiences result in impressions and we observe the impressions
• Proposes that causal relationships cannot be observed.
• Proposes that empirical knowledge is generated through the copy principle.

According to Hume, two repeated occurrences lead to associated impressions. This association is a correlation but is often interpreted as a causal relationship. It cannot be interpreted as a causal relationship as there are three criteria for a causal relationship:

1. Proximity of cause and effect
2. Cause precedes effect
3. Necessary connection between cause and effect.

It is impossible to observe a necessary connection (3). Induction refers to the generalization of observed cases to all cases. This is logically invalid, as the conclusion does not necessarily follow from the premises. The problem of induction became a driving force in scientific methodology and one of the results of this problem is Bayesian statistics.

Kant has several important characteristics:

• Proposes that causal relationships cannot be observed
• Proposes necessary and universal knowledge (e.g. mathematics).
• Attempts to synthesise empiricism and rationalism; he states that there is innate knowledge which is used to categorize perception (i.e. Kant’s glasses, experience is rationally loaded).

According to Kant, causal relationships arise from a mixture of rationalism and empiricism. The categories are universal principles of thinking, making a priori knowledge possible. The perception of reality is formed through observation (i.e. empiricism) and a priori knowledge (i.e. the categories which shape observation). Thoughts without content are empty and intuitions without concepts are blind.

Kant classifies disciplines into two categories:

1. Natural science
   This is lawful, quantitative description in the language of mathematics.
2. Historical doctrine of nature
   This is describing cases in ‘normal’ language and includes categorisation. It is solely empirical.

According to Kant, psychology falls under the historical doctrine of nature, as psychology cannot have explanatory laws and he believed that humans cannot directly observe their own observations due to the fact that the continuity of perception requires a stimulation that remains constant in time.

The attention of science moved towards whether ‘thinking’ can be studied scientifically, as it became clear that philosophical issues were rooted in the structure of the psyche in the early 18^th century. Reid proposed common-sense philosophy, stating that is it not necessary to be as sceptic as Hume and Kant, as we “know what we know”.

There were several reasons for the individualisation in Western society:

• Increasing diversity in occupations and complexity of social relations (Richards).
• Increasing control of the state on the individual (Foucault).
• Christianity putting emphasis on the solitary individual (Kagan).
• Improvement of mirrors, novels and postal services.
• Protestantism promoted devotional aids such as books and diaries (Pickren and Rutherford).