Why facts alone can never tell you how you ought to behave
Philosopher David Hume noticed a logical gap in moral arguments: writers often describe how the world is and then abruptly claim how things ought to be. Known as Hume's Guillotine, this principle states that you cannot derive a moral duty ('ought') purely from objective factual descriptions ('is'). To reach an ethical conclusion, you must always include at least one moral premise alongside your factual observations.
The Sudden Shift in Moral Thinking
In his 1739 work, A Treatise of Human Nature, Scottish philosopher David Hume made a quiet observation that would fundamentally reshape moral philosophy. Hume noted that in every system of morality he had encountered, authors invariably began with ordinary observations about human affairs, establishing facts about the existence of God or the behavior of people. Yet almost without warning, these writers would abruptly stop using copulas like 'is' and 'is not' and begin asserting propositions connected by 'ought' or 'ought not.'
Hume pointed out that this change occurs imperceptibly, but carries profound logical consequences. An 'ought' represents an entirely new relation, expressing prescription, duty, or value rather than description. Because deductive logic dictates that a valid conclusion cannot contain novel terms that were entirely absent from its premises, moving from purely descriptive premises to a prescriptive conclusion constitutes a logical leap that demands explicit justification. Because writers rarely provide that justification, the argument quietly breaks down.
This principle became known historically as Hume's Law, or more colorfully as Hume's Guillotine, named for the way it severs descriptive claims about the world from prescriptive claims about how people should act within it. It serves as a warning against assuming that because something exists in a particular state in nature or society, it necessarily follows that it should remain that way.
The Logic of the Fact-Value Distinction
At the core of the is–ought problem lies the distinction between descriptive statements, which report matters of fact, and normative statements, which evaluate or prescribe actions. Descriptive statements assert that something is true or false about the observable world. For instance, stating that human beings experience physical pain when injured is an empirical claim verifiable through biology and observation. In contrast, stating that one ought not to cause unnecessary pain is an evaluative claim.
Under classical deductive logic, an argument requires valid bridge premises to introduce new conceptual categories. If an argument consists solely of factual premises—such as 'climate change disrupts agricultural yields' or 'stealing destabilizes social trust'—it describes cause-and-effect relationships within the physical and social realm. To conclude that governments ought to curb emissions or that individuals ought not to steal, the argument requires an unstated evaluative premise, such as 'harming food security is bad' or 'social stability is desirable.'
Without that accompanying moral premise, the factual premises remain inert regarding obligation. Facts describe the terrain, but they do not supply the destination. Hume argued that human passions, desires, or moral sentiments provide the driving force behind moral judgments, meaning that reason and factual analysis alone cannot generate a moral obligation without an underlying value.
Moore's Open-Question Argument
In the early twentieth century, British philosopher G. E. Moore expanded upon the divide between natural facts and moral qualities in his 1903 work Principia Ethica. Moore introduced the concept of the 'naturalistic fallacy,' arguing that goodness cannot be equated with or defined by any natural property, such as pleasure, evolutionary fitness, or desire.
To demonstrate this, Moore formulated the open-question argument. If 'good' were truly synonymous with a natural property like 'pleasurable,' then asking 'Is something that causes pleasure actually good?' would be a closed, trivial question equivalent to asking 'Is a bachelor an unmarried man?' However, Moore noted that for any natural property proposed as the definition of good, it remains an open and intelligible question whether that property is indeed good. One can always coherently ask whether a specific pleasure is morally good.
Moore concluded that 'good' is a simple, non-natural, and unanalyzable property, much like the color yellow. Just as one cannot explain what yellow looks like to someone who cannot see it, one cannot define goodness in terms of physical constituents. While Moore's specific non-naturalist intuitionism differed from Hume's skepticism, both thinkers agreed that moral properties cannot simply be read off the physical features of the natural world.
Searle's Attempt to Cross the Divide
The rigidity of Hume's Guillotine prompted numerous philosophers to search for counterexamples where an 'ought' might logically follow from an 'is.' One of the most famous challenges came from philosopher John Searle in the 1960s, who argued that certain social realities, which he termed 'institutional facts,' contain built-in obligations.
Searle constructed a step-by-step derivation centered on the act of promising. He began with the purely factual premise that a person, Jones, uttered the words 'I hereby promise to pay you five dollars.' Searle argued that by definition, promising is an institutional act that places the speaker under an obligation. Therefore, from the factual act of uttering those words within the social institution of promising, one can logically deduce that Jones placed himself under an obligation, and consequently, that Jones ought to pay five dollars.
Critics of Searle, however, argued that his deduction still relied on concealed normative assumptions. They pointed out that the argument only holds if one accepts the institutional framework itself—namely, the evaluative premise that one ought to keep one's promises or uphold institutional rules. If an individual rejects the validity of the institution of promising altogether, the factual utterance of the words creates no binding moral duty, illustrating how difficult it remains to bypass Hume's restriction entirely.
Thick Concepts and Fact-Value Entanglement
Another major challenge to the strict dichotomy between facts and values arose from philosophers like Philippa Foot and Hilary Putnam, who argued that language does not always neatly divide into pure descriptions and pure evaluations. They highlighted the existence of what philosophers call 'thick ethical concepts,' terms that simultaneously describe behavior and convey moral evaluation.
Words such as 'cruel,' 'courageous,' 'honest,' or 'treacherous' carry descriptive content that is tied to specific factual circumstances, yet they also carry inherent normative weight. For example, describing an act as 'cruel' conveys factual information about the infliction of suffering while simultaneously evaluating that act negatively. Putnam argued that in ordinary human thought and discourse, facts and values are thoroughly entangled rather than separated into sterile, isolated compartments.
These critiques suggest that while Hume's logical point remains formally sound for purely non-evaluative premises, everyday human reasoning rarely operates with purely neutral descriptions. Our very categorization of the world and our descriptive language often already reflect evaluative choices, complicating the idea of a completely value-free factual starting point.
Practical Implications for Science and Society
The is–ought problem remains an essential concept in debates surrounding science, public policy, and ethics. A frequent misuse of scientific findings involves leaping directly from an empirical discovery about how human beings evolved or how nature functions to a conclusion about how society should be structured. For example, finding that certain behaviors offer an evolutionary advantage does not make those behaviors morally desirable or just.
Science excels at answering questions about what is: how diseases spread, how economies react to interest rates, or how ecosystems respond to warming temperatures. However, determining what society ought to do in response to those facts requires value judgments that science alone cannot supply. Deciding whether to prioritize economic growth over environmental preservation, or individual liberty over collective safety, depends on moral principles that must be argued on their own terms.
Recognizing the divide between is and ought does not diminish the importance of empirical facts; rather, it clarifies their role. Robust factual knowledge is indispensable for understanding the consequences of actions, but ethical values are required to decide which consequences are worth pursuing.
Key takeaways
•Hume's Guillotine asserts that a prescriptive conclusion ('ought') cannot be logically deduced entirely from descriptive factual premises ('is').
•To build a valid moral argument, at least one evaluative or normative premise must be included alongside the factual observations.
•G. E. Moore expanded this divide with the open-question argument, showing that moral goodness cannot be treated as identical to any natural property.
•While thinkers like John Searle and Hilary Putnam have challenged the rigidity of the boundary, the distinction remains fundamental in preventing the misuse of empirical facts as automatic moral mandates.