- Treatise on Human Nature by Hume


During the Enlightenment in the 18th century northern Europe scientific empiricism and revelation were engaged in a battle of the opposition between the physical and the metaphysical. Physics proposed a deterministic reality as the approach to understanding nature, replacing revelation. However that left philosophers without a goal since they did not employ the scientific method. This led to a return to Plato's epistemological question: how do we know? The answers were premised from a horizontal and a vertical worldview. George Berkeley, with extreme empiricism, affirmed that our perception of the world is built up of mental ideas that are rooted in a metaphysical conception of God as arch perceiver. Hume argued for physical explanations arising from two concepts of knowledge: ideas based on reason and calculation; facts based on sensory experience.

David Hume published his Treatise on Human Nature (1737) at the age of 26. He argued that all human knowledge can be divided into two categories: relations of ideas (mathematical and logical propositions) and facts (example: the sun appears in the east). Ideas come from our impressions or feelings. In contrast to the French rationalists, he said that even our most basic beliefs about the natural world cannot be established by reason, but we instinctively accept them - a clearly empirical point of view.

Central to Hume's thinking is the problem of induction: how can we conclude that observed objects behave the same as when they are not observed? He proposed as a solution that it is natural instinct, rather than reason, that explains our ability to make inductive guesses. However, the application of the inductive method in science suggests that it is based on a logical error because it assumes that the observation of now will be reproduced in the future when there is no observer. This forced Hume, a great admirer of empirical experience, to conclude that, although he recognized the logical fallacy, in practice science works.

Causality is another problem related to induction. Hume concluded that causality is a mental act of association, not a physical fact. If we are playing snooker and two balls touch, it is random. There is no determination, they just touch. However, a player who pushed one of the balls can interpret that he has determined the trajectory of the ball. He says he has caused the action. Actually, the cause is in his brain, not the ball. Hume criticized the acceptance of causality related to reality. His argument is that it is the brain that interprets what happens as causation, but that in reality the link between two events is accidental, not causal.

"Causes and effects cannot be discovered by reason, but by experience."  David Hume

In this way determinism would be an assumption, a construction of the brain to try to make sense of real chaos. In fact, the natural law would be randomness.

Hume entered the epistemological debate with his views on personal identity. He argued that we could not conceive of an apple without smell, colour, shape, taste, etc... otherwise it would cease to exist. This is similar to Berkeley's existence-in-perception. He applied the same argument to people and claimed that the person himself was nothing more than a cluster of interconnected perceptions. This contradicted the Cartesian concept of 'I think therefore I exist'. (Modern neurology agrees with Hume's vision.)

Like Hobbes, Hume attempted to reconcile human freedom with the determinist belief of physics. He proposed that freedom requires necessity. Furthermore, for us to be morally responsible our behaviour has to be either caused or necessary.

As for religion Hume's empiricism could not reconcile the idea of ​​God and sensory data. He argued that it was impossible to deduce the existence of a God from the existence of the world because cause cannot be determined by its effects. He left open the possibility of miracles that would be singular events different from the laws of nature. He added, with his traditional scepticism, that he did not know of such an event in history. (Modern medicine defines a miracle of physical healing as an event not yet explained by science.)

"No testimony is sufficient to establish a miracle, unless the testimony be of such a kind that its falsehood would be more miraculous than the fact which it endeavors to establish."

He opposed the argument of a designer God saying that there is design in nonsense things like snowflakes and therefore design does not prove intelligence. He also asked, "Who designed the Designer?"


Hume published three volumes of his Treatise anonymously : Books I and II in 1739 and book III in 1740. The aim of the work is to study human nature with the same rationality as Newton in the sciences. Hume argues against the contemporary rationalist view that reason is the basis for human beliefs. He uses scepticism about his own beliefs to counter this concept by demonstrating that emotion and mental habit are the real foundations of beliefs. He concludes that there is no rational basis for belief in morality, cause and effect or identity.

"It is not reason which is the guide of life, but custom."

- Book I: “Of the Understanding

The author starts by presenting the arguments for the validity of his empirical premise : all knowledge is based on experience. He applies this to a variety of philosophical ideas. The perceptive mechanism, according to Hume, is that we receive impressions through the senses then we form basic ideas about them and finally we elaborate complex ideas on that basis. Ideas are fundamentally similar to experiences. The philosopher goes on to define 'matters of fact'. These are not derived from reason or instinct but from experience. Hume concludes that metaphysical systems which do not use experience such as the existence of God, the soul and divine creation are not believable since we cannot base our proofs on direct impressions.

"Belief is nothing but a more vivid, lively, forcible, firm, steady conception of an object, than what the imagination alone is ever able to attain."

Two tools of Hume's philosophical analysis are presented in Book I : the microscope and the razor. The microscopic approach states that to grasp an idea we have to divide it into several simple ideas. If a simple idea is difficult to comprehend then we must repeat the impression it gave when first perceived. The razor affirms that if a term cannot be shown to be based on an idea which can be simplified then it has no meaning. He uses this concept to criticise abstract ideas relating to religion and metaphysics.

Hume offers another instrument for his analysis : the two-pronged fork. This is the principle that truths can be divided into two types: 

- The first kind is the relationships between ideas such as the mathematical truth that the sum of the angles of a triangle is always 180 degrees. This is a necessary truth since it does not change. 

- The other type of truth explains matters of fact which describe events in the world. The contrary of every matter of fact is still possible because it can never imply a contradiction:

"That the sun will not rise tomorrow is no less intelligible a proposition and implies no more contradiction that the affirmation that it will rise."

- Book II: “Of the Passions

"Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them."

Hume's classification of the passions is similar to his codification of impressions and ideas (Book I). He says that there are original impressions received through sensory experiences in the form of pleasure or pain. A secondary impression comes after an original one and emerges from it. The passions are placed in the secondary classification and are divided into direct passions like aversion, grief, joy hope and fear and indirect ones such as pride, humility, love and hatred. Hume also distinguishes between the cause and the object of passions.

Hume points out that moral decisions affect actions while those based on reason do not, so morality is not based on reason. It is pleasure or pain that are our motivations, not reason, since passions are not related to reason. Actions are moved by feelings even though they may be informed by reason. 

- Book III: “Of Morals

For Hume morality is based on impressions not ideas. He distinguishes moral impressions from common ones like sounds, tastes and colours. The impression of vice is pain; that of virtue is pleasure. Morality impressions also have their cause only in human actions, not in those of animals or inanimate objects. Morality is a social event since good or bad actions depend on how they affect others. In this way sympathy, a feeling for fellow humans, is the basis of moral obligation.

According to the author morality does not derive from experience and he gives the example of murder. This concept does not include the idea of vice but rather only dislike for the act. For this morality springs from the passions or feelings and not reason. Reason comes later to help explain the feelings but they do not originate in rationality.


Cause and effect

"There is a very remarkable inclination in human nature to bestow on external objects the same emotions which it observes in itself, and to find everywhere those ideas which are most present to it."

Hume allows that we perceive two events that apparently happen at the same time but he insists that we cannot know their relationship. He explains this through our mental habit of association which he thinks is an unfounded belief, not a fact, and as so meaningless. The author agrees that we have an instinctive belief in causality through our habits but that we cannot prove nor discount this belief. Religion presents God as the First Cause in a world which works on cause-and-effect but Hume describes this as unknowable.


"Even after the observation of the frequent conjunction of objects, we have no reason to draw any inference concerning any object beyond those of which we have had experience."

The procedure of generalising conclusions from specific experiences is induction. Aristotle used it to counter the prevalent deductive thinking of his teacher, Plato. It is basic to science and empirical thinking but it is uncertain since we may find data that disprove previous conclusions. The false premise of induction lies in the belief that we can predict the future based on the past, which is untrue according to Hume.

The author criticises two logical arguments sustaining induction. The first is that logically the future must look like the past. Hume argued that it is also easy to imagine a chaotic world therefore logic is no guarantee. The second is that events will continue to occur because they happened before. The philosopher discounts this as circular logic. However Hume recognised that we can still use induction in everyday life but we must realise that we know little about it.


Hume's idea of morality is that instead of being founded on God's will it is useful. He believed that we cannot justify morality as a scientifically justifiable solution to social problems. Moral principles are accepted because they are of social utility and help society to operate.

For Hume behaviour is not based on reason but passion. Its criterion is instrumental, that is it is at the service of the protagonist. However he observes that generally it does not always work this way and humans are motivated by actions other than their own best interest. He concludes that reason on its own cannot motivate people to act. Reason helps us to judge but it is our personal passions that direct us to act on or disregard these judgements. Rationality is an advisor not a decider. Immorality is decided not on the grounds of reason but of personal displeasure. Hume thus denies God's role as a source of morality.

The Orderly Universe

The philosopher rejects the claim that an orderly universe is proof of the existence of God. He notes that order is observable in mindless processes such as vegetation. He adds that even if the universe has a design we can know nothing of the designer and certainly not the divinity as religion portrays it. God, if he exists, does not fit the omniscient, powerful, beneficent criteria because evil exists.

The Self

"I may venture to affirm the rest of mankind, that they are nothing but a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement."

When observing ourselves we perceive stable characters that change little over time. However, our experiences are fleeting  sentiments and impressions, not a unified whole. We are only aware of momentary experiences, not of a 'self'. Memory allows us to connect our feelings and ideas but leaves no evidence of any essence. Hume proposes that the self is just an assortment of perceptions, links in a chain, so that to look for a core is to search for a chain without the links. Self, he argues, is the fruit of our habit of assigning sense to a collection of patterns. This is natural but not logical.

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