Philosophy of the mind: what are the thoughts of Descartes, Locke and Leibniz? - Chapter 2

René Descartes moved Paris in 1615, in the midst of an identity crisis. He did not think of his elite education as valuable because it had been too focused on the past. He also found it depressing that philosophy had never produced anything that was doubtful or uncertain. He became one of the first influential thinkers to have fully mechanical explanations for the traditional functions of the sensitive psyche or soul of Aristotle. Descartes described the human mind and body as two cooperating, yet different entities. According to him, both need their own analysis and explanation.

Who was Rene Descartes?

Descartes (1596-1650) was born in La Haye in France. He grew up with his grandmother. His father was a wealthy lawyer, but Descartes had little close ties with his family. His intelligence was noticed by his father, after which he sent his son to the most progressive school in France. 

Young Descartes learned about a science which was dominated by the Greek philosopher Aristotle (384-322 BC). The Aristotelian view of the universe placed the earth in the middle, surrounded by a number of rotating crystalline globes, namely the planets in our solar system, the moon, the sun and the stars. The Polish astronomer Nicolaus Copernicus (1473-1543) published a book in 1543, in which he assumed that not the earth, but the sun was the centre of the system. His view was not taken seriously, however. 

Descartes convinced his teachers that he could think best if he meditated in bed, allowing him to stay in bed all morning while other students were already working. When he left La Flèche at age 16, he was the best student of the best school in the country. He then migrated to Paris, and got influenced by Marin Mersenne, a French monk who offered Descartes intellectual and personal support. In 1618, he went into the army to see if the practical experiences of the "real world" would provide more satisfactory knowledge. The actual war had not started yet, so Descartes experienced seven months of boredom and learned quickly that the military had no more useful knowledge than scholars did. 

A turning point for Descartes was when he met Isaac Beeckman (1588-1637) in the Netherlands, a physician and internationally known mathematician. He became a mentor to Descartes and helped him to get back his intellectual interest. With his support, Descartes wrote his first scholarly work, an essay about music. When Beeckman had to leave Breda, Descartes went with him, and during the trip to the south he got two insights. According to a legend, the inspiration came when he saw a fly humming in the corner of his room. He suddenly realized that the position of the fly at any time can be exactly determined by three numbers, represented by the perpendicular distances from the fly of the two walls and the ceiling. Generalizing from this idea, he acknowledged that every point in space can also be defined by his numeric distances from randomly defined lines or planes. In short, Descartes had a means to unite and integrate the rather separate mathematical disciplines of geometry (with motifs) and algebra (with numbers). This was the foundation of the analytical geometry. 

This idea soon came enormous practical applications - for example, it would help astronomers to describe and calculate the orbits of planets. Given Descartes' great previous complaint about mathematics (the lack of practical utility), he certainly had a good reason to be happy with this invention. If he would have done nothing else in his life, he would still have had  an important place due to his analytical geometry in the history of science. 

He got his second insight in the German city Ulm. His vivid dreams were violent and full of panic, until a flash of light filled the room with rays and then his sleep became calm. He dreamed about a book with the phrase "which path in life shall I follow? " The book disappeared but came back with new and better engravings. According to Descartes this was the idea for a new method for obtaining true knowledge.

What was Descartes's method?

Descartes’ first rule of acquiring knowledge was to accept nothing as true unless it presented itself so clearly in his mind that there was no reason for it to be doubted. He doubted everything from that moment on. Descartes wanted to use systematic doubt to arrive at well-founded concepts which, like geometric axioms, would be the starting points for deductive reasoning in all non-mathematical fields. He became a man with a mission. 

He began by claiming that the most basic and fundamental properties of physical phenomena, which he called ‘simple natures’, of which existence can neither be analysed nor doubted. According to Descartes, there were only two simple natures; extension (space occupied by a physical particle or body) and movement . According to him, all physical phenomena could be explained in terms of these two properties. All physical properties are probably the result of elongated, material particles in motion.

Descartes also thought that living bodies can be seen as mechanical ingenuities, explainable according to the same principles. In the same period, the well-known  Italian researcher Galileo Galilei (1564-1642) published something very similar. In his work 'The Assayer 'from 1623, Galileo distinguished between primary and secondary characteristics of physical matter. The secondary properties of matter ( sound, smell and touch of one object) come after the primary properties (shapes, quantity and movement) to the human senses. The primary properties are inseparable from the matter. Our conscious experience of the world is secondary to, and of a completely different order than the elementary primary qualities that cause them. 

Descartes' first publication was titled 'Le Monde' (the world), containing his basic understanding of the physical layout of the universe, and the subjects ‘light’ and ‘vision’. Galileo was condemned by the Inquisition for encouraging the Copernican theory of the universe. Descartes’ first manuscript was for security reasons therefore only published after his death by his successors. His second work was "L'Homme" (the human), in which he presented his physical principles for analysing living bodies.

In "Treatise of Light", Descartes described his physical ideas, based on the analysis of said material particles in motion. Like Aristotle and unlike Democritus, Descartes believed that there could not be a void or vacuum, and so he thought of the entire universe as completely filled with different types of material particles in different forms of movement. When a particle moves, it does not leave an empty space behind, because the space is filled up by other particles. The three basic type particles in Descartes’ universe are the classical elements of fire, air and earth. Fire is seen by Descartes as unthinkable small, as 'a virtually perfect liquid', capable of filling up any space of any shape or size. He sees the element of air as slightly larger, but still too small to be able to observe directly. All material objects - including planets, comets and the earth with all objects on it – are presumably composed of the washing up of the earth particles. These are the third and heaviest variation in Descartes' hypothetical universe. As the original title of Le Monde, Treatise of Light suggests that most of this first treatise is about the phenomenon of light. The air particles naturally rearrange themselves into columns between objects, the material basis of forming light rays.

Descartes conceptualized the eye as a physical mechanism activated by the physical properties of light waves. From this point of view, he also described the structure of the living body, which according to him consisted of different physical systems that work on the hand of the laws of nature. He further developed this mechanistic view of the human body in the second, physiological part of 'Le Monde', the 'Treatise of Man'.

Descartes was not the first scientist to investigate human bodies in a mechanistic manner. Galileo examined the bones and joints of the body in terms of leverage-systems. William Harvey (1578-1657) analysed the heart as a physical pumping mechanism in a revolutionary demonstration. He showed that blood is not always is re-created, but that it is constantly circulating through the body. So, the actual contribution that Descartes delivered lies in the scope of his research, and not so much in the idea of ​​it examining the body as a physiological mechanism in itself. 

According to Descartes, all processes in the body are comparable to those of an automatic machine and he was mainly interested in the system of nerves. According to him, the brain contained ventricles filled with a clear yellow liquid, which he called "animal spirits" (known today as cerebrospinal fluid).

He mechanically analysed ten physical functions. He concluded that these bodily functions came into being mechanically. He then decided to replace the traditional concepts the vegetative and animal souls with mechanical explanations. Sensory stimulation in the form of vibrations that stimulate the sensory organs can be the nerves according to Descartes. As a result, vessels in the brain can be opened, allowing the animal spirits back to the nerves and muscles and glands can flow, causing movement or secretion occurs. Even though Descartes did not use the term, he formulated the idea for what we did now refer to the reflex: a neurophysiological sequence of activity through which a specific stimulus from the outside world automatically elicits a specific response in the organism. He distinguished between two types of reflex responses. The first one flows the vital souls directly along the nerve through which the stimulus came in, resulting in an automatic and immediate response. The second kind is the type of reflex that is responsible for learned reactions in which the response is not directly linked to the stimulus. This is similar to it current concept of the conditioned reflex. 

Descartes also experienced that internal factors such as emotions also play a role in animal reactions. He suggested that localized currents, whirlpools, or what he called disturbances, can develop in parts of the animal soul reservoir. These can thus influence the receptiveness of the nearby nerves. In this way, souls are directed towards the muscles. Because of such variations a person or animal might develop a proclivity or tendency to different emotions such as anger or fear. The reactions of a body are thus determined by a combination of external stimulation, and an internal 'emotional' willingness of the animal souls to respond in a certain way. Others mechanistic consequences of the animal souls are the states of sleeping or waking. Descartes saw the sleeping brain as relatively without soul, with weak tissues and weak nerve fibres with an inability to transmit the most external vibrations. A sleeping organism is therefore, in  general, non-responsive to external stimulation, with only a few isolated and decoupled experiences, or dreams, created by a momentary tightness in the nerves. During the wakeful hours, the ventricle is maximally filled with animal souls, causing it surrounding brain tissue and the nerves are highly sensitive to external stimulation.

According to Descartes, the difference between humans and animals was in human characteristics awareness and willpower. Actions can arise because they want this or because it follows from rational considerations. This didn’t relate to Descartes’ ideas of the mechanical body, but from the presence of a soul or spirit, which he thought to interact with the physical machine inside human beings. So he resigned from the Aristotelian concept of vegetative and animal souls, but retained the thought of the rational soul.

What are the rational qualities of the mind?

In the autobiographical 'Discourse on Method' Descartes described his first attempts on systematic doubt, meaning that everything is doubtful. Because of all this doubt, he eventually came with an idea of ​​which he was absolutely certain and what became one of his most famous passages: I think; therefore I exist (cogito ergo sum). His own rational thoughts, or soul, were an unquestionable reality. This led to the conclusion that thoughts are in contrast with the body. Ideas are independent of specific sensory experiences, even if they can be suggested by certain experiences. However, they arise from the nature of the thinking soul self. Descartes called this the innate ideas of the soul. 

The innate idea of ​​'perfection', in combination with his certainty of the reality of his own mind, also suggested to him that God had to exist, who embodied all aspects of perfection. For Descartes, there now were certainties of his conscious soul and a God. Because of this, this also meant that he could also trust the knowledge his senses provided him, because the integrity of the mind that it perceives and the perfection of the God who developed both matter and spirit were certain.

Descartes is considered a dualist because he sharply distinguishes between body and spirit. He also emphasized that many important phenomena are not the result of only the body, or only the mind. Instead, there are many possible types of interactions between the two. For this reason, Descartes' position is also called interactive dualism. Around 1640, Descartes went further into this dualism, mainly because of his friendship and correspondence with Princess Elizabeth or Bohemia (1618-1680). He used this correspondence as the foundation of his important work that he called 'Treatise on the Passions of the Soul' (1649). A body without a mind would, according to Descartes, be a like a vending machine, completely under the mechanistic control of external stimuli and the internal hydraulic or 'emotional' state, and completely without awareness. The body adds richness to the content of the consciousness of the soul, while the soul adds rationality and will power to the causes of behaviour. 

Descartes believed that the conscious perception accurately represented the real world. According to him, there had to be one specific location in the body where the double impressions of the eyes could become one unit, before they reached the soul. According to Descartes, this should be in the brain. He then learned about the pineal gland, a small structure near the centre of the brain, that ends into a large ventricle. Because this pineal gland was undivided, according to Descartes here the sensations of the divided body come in and become a unity before they are presented to the soul. The strategic location of the gland in the, with soul-filled, ventricle, also meant that the idea was to feel the commotion of the animal souls, which in his opinion were the cause of emotions. Descartes called these conscious experiences of this commotion the 'passions' - the conscious experience of feelings. First, it feels the specific nature of the movement of the gland, and therefore has a conscious sensation of a passion (a feeling like love, hatred, fear, wonder, or desire). Second, the soul can come from a conscious attitude towards that passion, trying to influence it by initiating voluntary movements in the pineal gland that strengthen or inhibit these emotional disturbances of animal spirit. For example, if the soul experiences anger, it can attack this offensive person by influencing the pineal gland to get even more souls in the nerves that encourage offensive reactions. Alternatively, the soul can inhibit the attack by the gland move, to block the flow of souls in those nerves. For relatively mild emotions, the soul may ignore or suppress these influences. This happens, for example, when mild background stimulation is suppressed when a person needs to concentrate.

In 1649, Descartes was invited by Queen Christina of Sweden to move to the North and become a philosopher at her court. This he did, and there he wrote frivolous verses to celebrate the life of Christina. Descartes died at the age of 53. He never married but became father of one daughter. She died, however, as a child. Descartes' basic ideas about the nervous system and the brain as a control centre that regulates behaviour turned out to be true, just like the general idea of ​​a reflex. Descartes' more general philosophies of mind have similarly been just as influential.

What is the difference between Locke and Leibniz?

Gottfriend Wilhelm Leibniz (1646-1716) and John Locke (1643-1704) were both philosophers. Leibniz was impressed by the book 'An Essay Concerning Human Understanding' that Locke had written. In it, Locke described the nature of human knowledge from an empirical perspective. Empiricists believe that knowledge has a sensory origin. However, Leibniz thought that Locke's empiricism went too far, and he did not see the role of several important properties in the mind that are innate. He tried to get in touch with Locke, but because it did not work, he worked out his ideas in a manuscript called 'Nouveaux Essais sur l'Entree Humain '(New Essays on Understanding Man), in which he supposedly went in dialogue with a representation of Locke. Locke died, however, just when the work was finished, and Leibniz did not want to discuss with a dead author, leaving his work only to be published half a century after his death.

Leibniz and Locke have many similarities and differences. A big similarity is that they both tried to integrate Descartes’s work with their political ideas of a larger and more general philosophy of the mind. Yet they both reacted differently to different aspects of Descartes' work. Locke accepted his basic ideas about physics and philosophy, while he rejected the idea of ​​a constant conscious thought with innate ideas. He supported the suggestion of Aristotle that the mind, at birth, is an unwritten book, that a person absorbs impressions from the external world and then collects them and reflects. Locke stated that all human knowledge comes from experience. Leibniz, on the other hand, rejected the physical aspects of Descartes on logical grounds. He agreed with the conscious soul of Descartes but concluded that the ultimate "substance" of the universe is a kind consciousness-bearing entity. He suggested a philosophy of the mind that emphasized the Nativist and rationalistic tendencies of Descartes.

Who was John Locke?

John Locke (1632-1704) was born in Wrington during the revolution. His father fought in the civil war of 1642, under the direction of Colonel Alexander Popham, the man who later sponsored the 15-year-old Locke, so he could get lessons at the famous Westminster School in London. In this tolerant environment Locke learnt that there are two or more sides to most statements and issues. After 5 years at Westminster, Locke won a scholarship at Oxford University. After Locke earned a title in classical antiquity, he studied very seriously with Thomas Willis and a handful of other progressive Oxford doctors and thus he became a competent doctor himself. Robert Boyle (1627-1691), is the founder of Boyle's law, which describes that the volume of a gas varies inversely with the top pressure. This was the basis for modern chemistry. Boyle helped Locke with opening a small scientific laboratory in Oxford, which led to a new kind of chemistry. During that time, Locke read Descartes' work, reinforcing his conviction that you should not just assume anything on the basis of authority. Locke's interest grew wider with the years and at the age of, almost, 34, he was successful in classical antiquity, medicine, science and diplomacy.

Sir Anthony Ashley Cooper (1621-1683) was a favourite of King Charles II and was appointed by him to be the first Lord Ashley, a baron, and then in 1672 'Earl of Shaftesbury'. He was on his way to become the most influential politician in the kingdom. In 1666 he met John Locke for the first time. The chronically ill Ashley was in in Oxford that year for the medicinal water they had there. The doctor who normally brought him that was sick and asked John Locke do it in his place. A friendship arose between Locke and Cooper. They both had a preference for tolerance in religion and a moderation of the government. Locke then became Cooper's personal doctor and moved to London when Cooper’s cyst became dangerous. Locke met the famous physician Thomas Sydenham and decided to perform a radical operation after which Cooper recovered again. Locke was now, besides a trusted politician, also a medical counsellor. 

Cooper became the leader of the eight developers and owners of the 'Carolina territory 'in the United States. His good friend Locke also had a lot of influence and played a major role in compiling the original fundamental constitutions, also called the legal code. This document contained a limited religious tolerance and a limited democratic right to vote. 

The Royal Society was a scientific organization Britain. Robert Boyle was already a member of it, and in 1668 Locke also became a member. He then came with the idea to explore the core of knowledge and thought. He wanted to discover what we could know with exact certainty and what not. He worked on this idea for 19 years and this led to his big book called 'Essay Concerning Human Understanding'. From 1679 on, there was a crisis in politics and in the monarchy about the succession of the English king. Shaftesbury didn’t want that his Catholic younger brother would take over the throne, which led to a quarrel between Charles and Shaftesbury. Shaftesbury was arrested and imprisoned in the Tower of London. Locke and Shaftesbury then both fled to the Netherlands. Once he was in the Netherlands, he continued working on his manuscripts ‘Essay Concerning Human Understanding’ and ‘Two Treatises of Government'. While he was doing that, the political climate in Groot- Britain had changed in such a way that it was then safe to publish these works. Charles II was succeeded by his Catholic brother James II, but after three years he was replaced by his Protestant daughter Mary and her Dutch husband, William of Orange. Locke then returned to England. Soon, thanks to his publications, Locke became the most famous philosopher in England.

What did Locke describe in 'An Essay Concerning Human Understanding'?

Locke believed that the recent discoveries of Boyle, Huygens and Newton represented the top of human knowledge. He adopted their observational and inductive methods as being ideal model for the best functioning of the human mind. In his essay, he presumed the existence of a human mind that basically operates according to the inductive model, and all its knowledge develops from observations of the external world. He saw the mind as receptive and often passive, and one of the primary functions would be to receive sensations. He therefore did not agree with Descartes, who stated that people have innate ideas that go deeper than the experiences that they have. Locke said that these ideas often cannot be found in inexperienced or weak spirits (he meant young children and mentally disabled). His conclusion was, therefore, that these ideas cannot be innate. He was therefore in favour of Aristotle's tabula rasa, the blank page. 

According to Locke, the human spirit has two kinds of experiences. First, it experiences sensations of objects the external world. Secondly, it experiences reflections of the operations of one's own mind. Such experiences produce representations or ideas in the mind, which do not only immediately take awareness, but also to remain in the form of memories. The inexperienced, earliest sensations and reflections of children evoke a number of simple ideas. Simple ideas of sensation are concepts such as redness, loudness, coldness or saltiness. With more experience simple ideas are combined by the mind in different combinations to create complex ones form ideas . For example redness, roundness and sweetness can be combined to the idea of ​​an apple. Locke states that all simple components of such ideas earlier have to be experienced. Without a concrete experimental basis of simple ideas, the clearest, 'true', complex ideas are impossible.

What kinds of knowledge did Locke distinguish?

Locke also looked at what human minds can know about the nature of ideas. He stated that knowledge "is nothing other than the perception of the connection and agreement, or difference and aversion, of ideas". A small number of perceptions are immediate and irresistible, such as the immediately recognized difference between black and white. Locke calls this ‘intuitive knowledge’. This is followed by demonstrative knowledge, which is obtained by stepwise, logical inferences, each part of which is intuitively certain, but the total pattern is not. Finally, he identified sensitive knowledge. This knowledge is acquired through certain patterns sensory experiences. It is this sensitive knowledge of which most of the human knowledge depends. Locke introduced the term association of ideas to it combine ideas. The first category of natural associations includes the redness and roundness of apples and the relationships in the scientific laws that were recently discovered by scientists like Boyle and Newton. The second category includes all accidentally connected ideas, such as by culture. Only the natural associations are true knowledge. Locke did not specify how ideas become associated. After his death, his successors introduced the terms 'law of association by proximity' and 'law of association by equality' around Locke's principles. Like Galileo and Descartes, Locke assumed a distinction between primary qualities that are inseparable from acquired objects, and the secondary qualities imposed on objects by sensations. The primary qualities are firmness, size, figure and mobility. Material objects in the world actually have these qualities and these form the image of the external world. Secondary qualities are sounds, colours, temperatures, tastes and smells. These characteristics arise from both the sensory organs as from the objects themselves. Locke considered these secondary qualities as less 'true'. Thus, secondary qualities had to be acquired to obtain true knowledge explained in the more basic primary principles.

What is the influence of Locke?

The essential message of the Essay was that all knowledge comes from experience, but also that no one's experience can be sufficient to gain a complete and correct knowledge of the world. This was in line with the new British political climate. Because no one could claim exclusive access to the truth, there was tolerance for religious questions and participation in public affairs was encouraged. Locke described these implications in his second big book in 1690 called 'Two Treatises of Government'. In this book, he applied the theory of social contract and he further elaborated the theory. This theory was earlier introduced by Thomas Hobbes (1588-1679). Hobbes regarded people as naturally aggressive, self-centred and predatory. This led to our ancestors forming a group with centralized power so that they could organize resistance against other groups and combat aggression within their own. Hobbes found that for survival, compliance with the centralized authority was necessary, regardless of the form that this authority. Locke also saw that rulers and their subjects were tied together by one implicit social contract, but he had a more positive view of humanity in its natural status. He stated that people have the opportunity to gain more valid knowledge from their own experiences. In doing so, they can benefit from the combined experiences of groups people. He viewed the arrangement of the social contract as a rational choice, in which real benefits are brought in by individuals, by investing in protective and regulatory functions of a centralized authority. When the government defends the interests of the violated people, these people have the right to be heard, to revolt and / or appoint a new authority. In 1693, Locke published a short work in which he argued for education based on experiences and scientific observation. He also produced four revised versions of his Essay.

Who was Leibniz?

Gottfried Leibniz (1646-1716) was born in Leipzig. At the age of fourteen, he was accepted into the University of Leipzig. He then worked briefly in Nuremberg,  and migrated to Mainz, where he gained a position as adviser to the Prince-Elector. In his early years as adviser to the elector, he worked on the development of a new one method for teaching the subject of law, a system for libraries, and a system for the review of school textbooks. He also began to study the history and culture of China. 

In 1672 he was sent to Paris as a diplomat. Leibniz became interested in mathematics and had three important contributions in this field. The first was mechanical: he designed a calculator, a predecessor of modern computers. His second contribution was the description of binary geometry, the representation of all numbers with only 1s and 0s. This became the basis of count in electronic computers. His third contribution was the 'infinitesimal calculus'. Beside the scientific and practical importance of this, the calculus suggested two important ideas for Leibniz 'philosophy. First, the principle of constant and continuous change. Secondly, the calculus was mental fiction, and could not be concretely observed in reality, though it did reflect reality. 

In Amsterdam, Leibniz had two important experiences. First he met the philosopher Spinoza (1632-1677), a brilliant Jewish student who was a pantheist – the notion that God is the universe. Secondly he met Antonie van Leeuwenhoek, who developed the modern microscope.

The new duke who served Leibniz in Hanover was married to Sophie the Countess Palatine (1630-1714), the youngest sister of the confidant of Descartes, Elizabeth of Bohemia. Sophie became Leibniz’ friend and supporter, and her daughter Sophie Charlotte joined her. This mother-daughter team became the first audience for Leibniz' philosophy and his letters to them became the basis of his important work. Leibniz was open-minded and not ethnocentric and maintained correspondences with China. He published reactions from China in a book 'News from China'. He found that China and Europe could learn a lot from each other.

Leibniz died when he was 70. Half a century later, publications from his manuscripts and private pieces revealed the true extend of his genius, just like a philosophy of mind that led to a scientific psychology in Germany. His most important psychological work was the extensive response to Locke's Essay, the 'New Essays' on human understanding. He had previously communicated these ideas in his correspondence with the two Sophies, and in a short work, De Monadology.

Leibniz looked at populations of microorganisms in a pond through Van Leeuwenhoek’s microscope in Amsterdam. He saw the universe as a hierarchy of living organisms within other, larger organisms. He did not agree with Descartes and said that despite the fact that a living body is some kind of divine machine, it surpasses any artificial automaton because it isn’t made up out of dead matter but of other living organisms, which again have other living parts contain, to the infinite. He also did not agree with Descartes and Locke that the fundamental units are dead, material particles that move and interact with each other. According to Leibniz, one can never come to a small piece of matter and say that that's the real thing. This also applies for movement. The ultimate units of the world are, according to Leibniz, dynamic entities that are energetic and purposeful, with some capacity of consciousness. He called these units monads.

Leibniz also believed that monads should differ in their capacity of consciousness and made a hierarchy of four classes. The uncovered monads had only a small capacity of consciousness, similar to someone in deep, dreamless sleep. A level higher were the feeling monads, with capacities for conscious sensation and perception of material objects and for the memory of these experiences. When they were merged, it became the soul of an animal. Even higher were the rational monads, which can form the soul or spirit of man. Leibniz also called consciousness apperception, in which an impression or idea is not alone observed, but also further interpreted in terms of underlying principles and laws. Apperception also includes reflexivity, the subjective feeling of the self. Standing at the top were the supreme monads, equal to god. In summary, Leibniz' universe was more of an organism than that of a mechanism. Each monad had its own purpose but were coordinated by larger goals and the consciousness of a supreme monad.

Leibniz was a supporter of Plato's nativism. He called all innate ideas and predispositions 'necessary truths' and he saw his ideas as additions to Locke's points. Locke saw sensations from the outside world and subjective reflections on his own mind as the two sources of ideas, but did not say much about the reflections, while Leibniz did. In some points, Leibniz summarized what Locke had already said. Locke however, claimed that the mind is not constantly active. According to Leibniz, the mind was always active, even during dreamless sleep. However, you are not always aware of this activity. Leibniz called this 'minute perceptions' - they are real, but do not enter consciousness. Sometimes we do become aware of our minute perceptions, for example when we move our attention to background noise. Usually, however, they remain in our subconscious. Leibniz thought that minute perceptions also play a role in motivation, because they would determine our behaviour in many situations, even when we do not think about them. So, according to him, we would never be indifferent to a situation. The difference between Locke and Leibniz was mainly due to their different goals. Locke wanted to discover the limits of knowledge and establish new rules for solving political and daily issues. He was primarily an empiricist. Leibniz was more of a nativist and saw the active spirit as a primary subject of interest.

​​Locke’s way of thinking was especially influential in English-speaking countries, as he was the founder of British associationism. Berkeley applied his idea of ​​association principles to the systematic analysis of depth perception. A century later, David Hume helped the know-how association by formalizing contiguity and equality. Hume's fellow scholar Hartley (1705-1757), claimed that ideas are the subjective results of very short vibrations at specific locations in the brain that become connected. Later, in the 19th century, James and Stuart Mill claimed that the most important differences in character, behaviour and intellect come from associationist principles, meaning by differences in experiences and associations. Galton did not agree with that, which became the start of the nature-nurture debate. In the 20th century, the ideas it associationism and Locke came together in what then became behaviourism. Leibniz’ theory was more dominant in Europe, for example with Kant and Wundt. You can also see his ideas return in Freud's psychoanalysis and Piaget’s developmental schemas.



  • You will find Descartes' thoughts and ideas in every other pioneers' thinking. It is crucial that you understand his theories.
  • Descartes did make a major flaw when it comes to finishing his ideas. He often makes a very true, rational start, and then came up with illogical conclusions to fit his thinking. Try to find that line within his theories.
  • Try to connect these philosophers to Socrates, Plato and Aristotle. How did these new theories for out of the old ones?
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