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David Hume, the radical empiricist who knocked on Existentialism's door


December 25, 2020

14 min read

Physical space doesn't exist; it is only a construct of the mind, created out of experience.

Time doesn't exist either. It is also a construct of the mind, arising from collected experience rather than the nature of "reality." So is the notion of cause and effect, as well as the sense of a persistent self.

These are some assertions that arise from the framework of empirical rationalism presented by 18th century Scottish philosopher David Hume, a monumental figure of enlightenment-era philosophy. Hume railed against abstract ideas derived from "fancy" rather than experience and understanding, aspiring to clear away the "sophistry and illusion" of traditional metaphysics and philosophy. His work directly inspired such philosophies as utilitarianism and such philosophers as Kant, who credits Hume for "[interrupting] my dogmatic slumber."

Personally, reading Hume was exciting because of connections between his ideas and philosophies I'm more familiar with that came after him, especially Existentialism and Absurdism. Hume's deconstructions of misguided abstractions on reality are echoed in Kierkegaard's Individual/Crowd and Despair a century afterwards, and in Sartre's Bad Faith and Anguish a century after that. Existentialism and Absurdism can be framed as further interrogations of unresolved, or even previously unasked, questions arising from Hume's philosophy (specifically, after arriving at absurdism, using Hume's new empirical and rational, even scientific, dialectic: what is the meaning of life?). It was thrilling to read Hume, then, as a foundational predecessor to the more modern philosophy I'm more familiar with.

With that in mind, let's dive into Hume's radical empiricism, going over how it tore down the philosophies of old and ushered in the modern era of investigating the nature and meaning of existence.

Impressions and ideas

Hume divides mental perceptions — that is, all the ways that we as thinking, feeling beings perceive our existence — into two categories: impressions and ideas.

Impressions stem from external sensations and internal feelings. For example, you may see a rose and experience an impression of that rose. Or, you may feel sad, and experience an impression of this sadness. Complex impressions are built from simple ones: the impression of a rose is not a monolithic impression, but rather made up of component impressions of color, texture, shape, etc.

Hume's "Copy Thesis" then states that all other ideas stem directly from these impressions, i.e. every idea that one has can be traced back to impressions, and their corresponding external sensations or internal feelings.

So far, this framework seems par for the course of empirical rationalism: Descartes' ontology of mind and matter come to mind. It's Hume's next claim, however, where he breaks away from Descartes and the metaphysics of old.

There is no tree

Hume's "Liveliness Thesis" states that there is no fundamental difference between impressions and ideas. The only difference is in their "liveliness": one's impression of a tree is more vivid than their memory of the tree.

This equation of ideas and impressions, of mind and matter, has repercussions in both directions. For one, mind has been brought down to the level of matter. The operations of the mind are physical and mechanistic, arising solely from the inputs that it is given, lacking any of the sanctity or spirituality that thinkers like Descartes still held on to. Hume even goes so far as to say that our thoughts and decisions are deterministic, in that what we conceive of as free will and thought is only a complex, mechanistic amalgamation of our experiences.

In the other direction, Hume's Liveliness thesis leads to a rejection of the notion of an abstract reality itself. For example, if we see a tree, we experience an impression of that tree, and conclude that the tree must physically exist in some way beyond the realm of mind. If we walk a bit and then see a road, conclude that the road also physically exists, and lies some physical distance from the earlier perceived tree.

Because impressions are nothing more than ideas, however, Hume says that we have no valid claim that the tree physically exists. All we have is our impression of the tree: we do not know where that impression came from. The idea of physical existence itself is a construction of the mind, created to make sense of all our impressions.

In a similar fashion, the claims at the beginning of this blog post are arrived at. Time, that seemingly unceasing, immutable thing, we similarly have no claim really exists. All we can say is that we experience impressions and ideas in a certain way, and to make sense of those impressions and ideas we come up with the concept of the passage of time.

Is does not imply ought

These claims constitute Hume's attack on philosophical thought that rely on unfounded leaps from the experienced to the abstract. The idea of a soul — even Descartes' I think, therefore I am — is one such example. Hume labels such leaps of logic as claims that "is implies ought," which Descartes' cogito argument fits neatly into. The observation that "I think" is sound, but the conclusion "I am" is entirely unfounded: what does it mean to be? Where did this idea of "being" come from?

Hume rejects the idea of a persistent self altogether: you only experience yourself, as a collection of ideas and impressions, in each moment. The idea that any sort of abstract entity of self exists, and persists and changes with time, is a fabrication of the mind in response to the ideas and impressions it experiences.

Claims of cause and effect itself is often framed as abstract truth, when it can only be said to be a matter of probability, Hume asserts. When we observe a billiard ball striking another, and the struck ball moving, we cannot metaphysically declare that the striking ball physically caused the struck ball to move, only that these two events follow each other and we expect the same to occur in future, similar situations. Again, this is a rejection of a claim that "is implies ought," i.e. a cause and effect observed together, over and over, does not imply that there ought to be an abstract phenomena that exists to explain these experiences.

Imagination and understanding

If we cannot declare a billiard ball striking another to be the cause of the struck ball moving, then, what of physics? Does Hume's philosophy lead us to a rejection of all pursuits of understanding the world around us?

Far from it. In fact, Hume was a big fan of Newton, and modeled his own philosophy after him. Hume tore down is-ought philosophy not for the sake of tearing it down, but for clearing the way for newly clarified, enlightened thought.

What then, to Hume, counts as metaphysically valid thought?

Hume's Fork

To answer this question, let's revisit Hume's breakdown of mental perception from earlier. Recall that all perceptions can be split into ideas and impressions. Hume further splits ideas into memory and imagination. Memories are straightforward: they are recollections of other impressions and ideas. Imagination arises from the breaking down and recombination of other impressions and ideas.

Hume provides two categories within imagination that encompass sound thought, which he labels "understanding," dismissing all else as superstitious "fancy."

The first category of understanding encompasses "matters of fact." Earlier, Hume rejected assertions of abstract fact — but they are only really problematic when they are treated as certainties. To Hume, conceptions of physical reality are useful when they are treated as judgements of probability. "The sun will rise tomorrow," for example, is a faulty claim if used as evidence to support the existence of a Sun God who causes it to rise each day. It's a valid claim, though, with recognition that it is only an observation, or a collection of observations, of something that "is" or at least has been in the world.

These are the very limitations that the pursuit of "science" — in Hume's day, natural philosophy — is built upon. Theorems and laws are fundamentally not assertions of the nature of reality, but a continuously refined model, rigorously verified through repeated observation rather than empty conjecture.

Hume's second category of understanding encompasses "relations of ideas." This is akin to mathematical reasoning, for example the geometric proof that gives rise to the theorem "the squares of the legs of a right triangle equal the square of the hypotenuse." These relations are "discoverable by the mere operation of thought, without dependence on what is anywhere existent in the universe," Hume writes.

Revisiting the realm of science, and specifically Newton's laws of motion: "matter of fact" perceptions may show Newton that certain objects move in certain ways when colliding with other objects. Measurement may give Newton specific numbers to work with. From these numbers, "relations of ideas" allow Newton to derive equations for the relationship between force, mass, and acceleration, for gravitation, etc.

Thus, Newton's laws of motion, and other such rigorously-made scientific and philosophical claims, escape the unfounded is-ought leaps of prior thought. Hume's two categories of "matter of fact" and "relations of ideas" is often referred to as "Hume's Fork," and he uses them as the basis for skepticism and construction alike. From the ending of Hume's book An Enquiry Concerning Human Understanding:

If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.

Reason is the slave of passion

For all that Hume defined useful and unfounded philosophical reasoning, he returns to the idea of passions when discussing individual decision-making and morality.

Human actions are motivated purely by passion, Hume writes, with reason only emerging from and driven by passion and emotion. The full version of the quote in the section header: "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." (A Treatise of Human Nature) Emotions are experienced as impressions, after all, on which all further ideas are based — further ideas of which reason is only a subset.

Moral sense theory

This emphasis on emotion is clearly reflected in Hume's construction of morality. Moral decisions cannot be arrived at by rational deduction, but is rather rooted in emotions, a sensation just like any other perceptory sense.

Three subjects are required for the judgement of morality, according to Hume: the moral agent, the receiver of the agent's actions, and the spectator. The spectator may be the same person as the moral agent or the spectator, or a third person. Based on the moral agent's actions, and the impact that they have on the receiver, the spectator makes a judgement about the moral qualities of the agent. Rather than the judgement of whether or not an action is moral being determined by a logically crafted system of ethics, it ultimately comes down to the spectator's sympathy for either positive or negative impacts on the receiver of the agent's actions.

Hume classifies qualities of virtue and vice into four irreducible categories:

  • Qualities agreeable to others, e.g. wit, eloquence, cleanliness
  • Qualities agreeable to oneself, e.g. good humor, self-esteem, pride
  • Qualities useful to others, e.g. benevolence, meekness, charity, justice
  • Qualities useful to oneself, e.g. industry, perseverence, patience

Agreeable traits are ones which bring the spectator pleasure without "any further thought to the wider consequences that trait brings about." These are the perception-like innate senses of virtue and vice, applied deontologically, i.e. purely in consideration of principle rather than impact.

Useful traits lean into the realm of utilitarianism. Hume ties useful virtues to having an overall benefit on society: a consideration of the general impact of actions according to some internally defined function.

Themes of utilitarianism are even more strongly present in Hume's advocacy for the usefulness of "artificial virtues," like justice, property, and lawfulness. Though these are designed abstractions upon natural moral virtues, they are valuable because they serve the good of humanity and society. In this assertion, perhaps, we see hints of Hume's work as a historian and political commentator (for which he was even better known than his philosophy while he was alive) coming through.

Beyond Hume: what Is the meaning of Life?

Hume's philosophy centers experience — i.e. impressions of sensation or emotion — and reasoning — i.e. logical or probabilistic ideas made from impressions — in our understanding of reality. This framework sweeps away systems of metaphysics that rely not on experience or reasoning, clearing the way for science (i.e. more rigorous philosophy in general) and utilitarianism. This framework of skepticism leaves many questions unresolved, however: why do we feel what we do? If we cannot know that anything exists outside of our minds, how do we find meaning? On what principles should we live our lives?

Hume doesn't seem to dive into these questions in his work. He is weary of previous philosophers giving completely unfounded answers to these questions, after all; to clear away the previous conclusion and allow oneself to live by one's emotions, recognizing that no objectivity or rationality is of higher importance, is already quite profound and meaningful.

Suppose we are not satisfied with this, however. We live in the 21st century, where a large chunk of humanity have rejected is-ought metaphysics in favor of tempered scientific reasoning since childhood, where "reason is the slave of passion" is an interesting quote but not a source of revolutionary meaning. Once we've cleared away all the dogma and live in a science-driven intellectual utopia, have we really found more meaning or understanding of existence at a personal level than before? We shifted from knowing nothing about the nature of our existence but deluding ourselves that we do know something, to knowing nothing about the nature of our existence and acknowledging so.

Here, Existentialism takes up the mantle.

Despair Is the Sickness unto Death

As I wrote the above lines about knowing nothing about our existence, I surprised myself by how similar to Kierkegaard's language it sounded ("we very often reason from, rather than to, our convictions," The School of Life names as a phenomena Hume paid attention to). Writing in the 19th century, Danish philosopher Soren Kierkegaard is considered to be the first existentialist philosopher, predating Nietzsche by several decades and Sartre by a century.

Like Hume, Kierkegaard attacked existing dogmas and false conceptions of reality and meaning, even more ferociously than Hume. He names the state of being confused about, or having false beliefs about reality and meaning "despair." The lowest state of despair, other than not being able to despair (i.e. not having self-awareness), is to believe mindlessly, dogmatically, in something. To believe that material wealth, or relationships, or any form of worldly pleasure brings life meaning and happiness — that is an illusion masking an underlying despair of not knowing, and never having confronted, what the meaning of life really is.

The more that the meaning of life is questioned, the higher the level of despair. Simply being upset at the state of your life, like hating your job or being sad over a breakup, brings you one level higher, because even though you are deluding yourself about the source of your despair, you do not delude yourself that you are not experiencing any. Rising levels of despair eventually reach a point of intense questioning and rejection of existing meaning — for example, when one has a mid-life crisis and questions what makes them happy and what they are living for, making uncharacteristic decisions and purchases to break away from their mindless past.

If this questioning goes deep enough, it will inevitably lead to confrontation with the fact that life is absurd. The idea of absurdity isn't necessarily that life has no meaning, or even that we have no creator: rather, it's the fact that, because we are trapped in the mortal subjectivity of our existence, we can never know the meaning or nature of our existence. If one believes in God and is committed to following His path for them, they run into the problem: how do they know what God's path for them is? How do they know if they are on it? Even if an angel descended from heaven and whispers to you God's message, in the Humean dialectic, we cannot soundly draw ought from is: we have no truly rigorous way to assert that this angel is truly an angel, who represents God, because these concepts are outside the realm of our subjectivities and we may never verify them no matter how much experience we accumulate.

The idea of the absurdity of existence is borrowed from 20th century philosopher and novelist Albert Camus. Kierkegaard incorporates absurdity into his framework for despair and meaning. The most famous Existentialist philosopher, Jean-Paul Sartre, builds off of absurdity and constructs the concept of anguish: the realization that this grand idea we think of as the Meaning of Life is a construction entirely and solely in our own, individual power to shape.

Believing in ought

Hume's skepticism accomplishes much the same metaphysical slate-cleaning as Existentialist and Absurdist philosophers carried out a century later. The attitude towards the same metaphysical ideas by Hume and later philosophers, however, were profoundly different.

While Hume was satisfied with his arrival at absurdism, Existentialists were terrified by it. This is reflected in the names they gave (despair, absurdism, anguish), and in the importance placed on further interrogating what to do once confronted with absurdism. In fact, the question of what the meaning of life is — whether or not it is worth living, and how it should be understood — is taken as a uniquely important one, to which all others are secondary:

"There is but one truly serious philosophical problem and that is suicide. Judging whether life is or is not worth living amounts to answering the fundamental question of philosophy. All the rest — whether or not the world has three dimensions, whether the mind has nine or twelve categories — comes afterwards. These are games; one must first answer." — Albert Camus

To that end, Existentialism does actually provide an answer!

Hume rejected belief in religion as dogmatic and unfounded. Kierkegaard would likely agree, for the most part. But unlike Hume, Kierkegaard was not an atheist. In fact, Kierkegaard came up with the fundamental ideas of Existentialism for the first time in an effort to provide a justification for Christian faith.

Kierkegaard's key idea is the "leap of faith." To believe in Christianity, or any sort of religion or meaning in life, after fully confronting the total inability to truly ever know the nature of one's existence, is not something that can be arrived at rationally. Rather, it must be a purely irrational, illogical commitment to believe in something. This commitment differs from believing in an abstract notion of meaning, i.e. the kind of metaphysics Hume worked to tear down, because it is made with full confrontation of the futility of trying to assert "ought" from "is" rather than avoiding this confrontation. To truly escape from despair and find meaning in life, Kierkegaard asserts, one must irrationally believe that something "ought" to be.

In Kierkegaard's case, what ought to be was Christian religion and theology. The Existentialists who followed Kierkegaard were largely atheists (Nietzsche, Sartre, Camus, etc.), but they espoused the same idea that believing in "ought" is the step to the highest level of meaning possible.

Nietzsche expresses this in the idea of the Ubermensch, a person who defines their morals and beliefs about the world purely at an individual level, rejecting all historical and external dogma. The necessity, arbitrariness, and power attached to this self-definition of meaning constitutes Sartre's idea of Anguish.

Camus illustrates the context and profundity of such a self-definition in his famous essay "The Myth of Sisyphus". Life is just as meaningless and absurd as the punishment of Sisyphus, who is condemned to roll a boulder up a slope, only to have it roll back down the slope every time, for eternity. Yet, within this meaninglessness and absurdity, we must find meaning and happiness. "One must imagine Sisyphus happy," reads the last clause of Camus' essay.


I've always felt like Existentialism and Absurdism felt fundamentally solid in a way that other philosophies didn't. Granted, I haven't studied other philosophers or areas of philosophy deeply, but even the fact that Existentialism is what turned humanities-rejecting me into a philosophy and literature nerd speaks to how true and pertinent it feels.

Existentialism has served as my grounding point as I build my philosophical knowledge, a reference point to compare new philosophies and knowledge to. My reading of Hume has been no different.

Hume has been more exciting than previous such reads like Descartes and even Nietzsche because it connects so directly, while supplying another layer of historical and philosophical understanding. I can now express the solidity of Existentialism in with the vocabulary, as well as the name and body of work, of Hume's empirical rationalism, and also view Existentialism as an evolution on the ideas of Hume and what came before.

In my last blog post about Descartes, I pondered the progression of philosophical and metaphysical knowledge over time. Humans are continuously dissatisfied and seek more (Dostoevsky labels this the chasing of Palaces of Crystal, Kierkegaard would say we're caught up in unidentified despair). Descartes sought to move the line between physical and metaphysical back a notch, between matter and mind. Hume moved this line back a notch further, and more importantly outright rejected traditional thought that took place on the metaphysical side of the line as unfounded and dangerous. Existentialism moves the line back even a notch further: contemplations of metaphysics and meaning doesn't have to be unfounded as traditional philosophy has been, but can be carried out with the same rigorous dialectics as used by Hume.

Today, the outcomes of this Existentialist dialectic feel...mostly convincing to me. They feel coherent and true, profound and thought-provoking. But there's an element of dissatisfaction. At one point I said to a friend that, in my understanding of Kierkegaard's framework for despair, I had taken a leap of faith into the philosophy of Existentialism. On further reflection, though, I've realized that I haven't even started the true, individual questioning process. My philosophical thought manifests primarily through dogma, following the ideas of the crowd. It will take much more learning and thought before arriving at any sort of true belief.

There's the knowledge, too, that there's more out there. Hume's dialectic was satisfactory in his day, but was found to be dissatisfactory and expanded upon shortly after his career. Existentialism has similarly been around for 150 years now, and out of dissatisfaction with the Existentialist dialectic new ideas like phenomenology came to be. What lies beyond Existentialism? What lies beyond phenomenology? Will there ever be a singularity point, or some point of maximum understanding about the nature of existence, that we will no longer be able to push beyond, or will this cycle continue endlessly?

These are the questions I'll have in mind as I continue my reading of philosophers past 😁

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