Thomas Reid
Based on Wikipedia: Thomas Reid
David Hume had a problem. Not with his arguments—those were brilliant, devastating, and seemingly airtight. The problem was that his own philosophy told him he couldn't actually know whether the desk in front of him existed, whether the sun would rise tomorrow, or whether the friend he was about to have dinner with was anything more than a bundle of sensations flickering through his consciousness. Hume knew this was absurd. He admitted as much. But he couldn't find the flaw in his reasoning.
Thomas Reid could.
Reid, a Scottish minister turned philosopher, became Hume's earliest and fiercest critic—though "fiercest" might be the wrong word for a man who so admired his opponent that he sent him an early manuscript of his rebuttal through a mutual friend. Hume responded graciously, calling the work "lively" and "entertaining," while also noting some methodological quibbles. This was 18th-century intellectual combat at its most civilized: two brilliant minds dueling over whether reality was real, exchanging compliments between thrusts.
The Minister Who Challenged the Skeptics
Reid was born in 1710 in the manse at Strachan, a parish in the Scottish countryside of Aberdeenshire. A manse is the residence provided for a minister, and Reid's father Lewis was the local clergyman—setting a pattern his son would initially follow. The family had intellectual pedigree: Reid's mother Margaret was first cousin to James Gregory, part of a dynasty of Scottish mathematicians and scientists.
At thirteen, Reid entered the University of Aberdeen. By twenty-one, he was licensed to preach by the Church of Scotland. For the next two decades, he served as a parish minister, a respectable if unglamorous career path. But Reid was thinking harder about philosophy than about sermons.
In 1752, everything changed. He received a professorship at King's College, Aberdeen, which meant leaving the ministry behind. He founded the Aberdeen Philosophical Society—locals called it the "Wise Club"—where intellectuals gathered to debate ideas. And he began writing the book that would make his reputation.
The Inquiry That Changed Everything
An Inquiry Into the Human Mind on the Principles of Common Sense appeared in 1764. Almost immediately, Reid was offered one of the most prestigious positions in the academic world: the Professorship of Moral Philosophy at the University of Glasgow. He was replacing Adam Smith—yes, that Adam Smith, who was leaving to pursue other projects (he had a book about wealth and nations brewing).
For seventeen years Reid taught at Glasgow, developing his ideas in lecture form. After retiring in 1781, he spent the remaining fifteen years of his life converting those lectures into two major works: Essays on the Intellectual Powers of Man in 1785 and Essays on the Active Powers of the Human Mind in 1788. He died in 1796, having outlived his wife Elizabeth (his cousin) and all but one of his children.
The Problem Reid Was Solving
To understand what Reid was up to, you need to understand the philosophical crisis he was responding to.
For over a century, European philosophers had been wrestling with a fundamental question: How do we know anything about the world outside our own minds? The dominant answer, developed by René Descartes, John Locke, and others, was called the "Theory of Ideas." It worked roughly like this:
When you look at an apple, you don't perceive the apple directly. Instead, light bounces off the apple, enters your eye, triggers nerve impulses, and your mind constructs an "idea" or mental representation of the apple. What you're actually aware of is this idea, this internal picture. The real apple is out there somewhere, but you only ever experience your mental copy of it.
This seems reasonable enough. After all, we know that perception involves complex neurological processes. We know that optical illusions exist, that colors look different under different lighting, that our senses can be fooled. The Theory of Ideas accounts for all of this by putting a layer of mental representation between us and reality.
The trouble is where this leads.
George Berkeley, an Irish philosopher and bishop, pointed out an uncomfortable implication: if we only ever experience our ideas, how do we know there's a material world causing them at all? Maybe there's nothing out there. Maybe what we call "physical objects" are just particularly stable and consistent ideas in our minds—ideas placed there by God. Berkeley was serious about this. He thought he'd proved that matter doesn't exist.
Then came Hume, who pushed even further. If we only have access to our perceptions, Hume argued, we can't know whether the external world exists, we can't know whether other minds exist, we can't even prove that the self exists as anything more than a bundle of passing impressions. Causation itself becomes suspect—we never actually perceive one thing causing another, just one thing following another. The foundations of human knowledge dissolve into radical skepticism.
Hume was too honest to pretend this didn't bother him. He famously wrote that after his philosophical investigations left him in despair, he would go play backgammon with his friends, and the skeptical doubts would fade. But he couldn't refute them intellectually. They seemed logically airtight.
Reid's Revolutionary Move
Reid looked at this whole tradition and said: you've made a mistake at the very beginning.
The mistake was the Theory of Ideas itself—the assumption that we perceive mental representations rather than things themselves. Reid called this "direct realism" or "common sense realism." When you see an apple, you're seeing an apple, not an idea of an apple. Your mind doesn't construct a copy of reality; it opens directly onto reality itself.
This might sound naive. Aren't we just ignoring all that neuroscience about nerve impulses and mental processing? Not at all. Reid drew a crucial distinction between sensation and perception.
Sensation is the raw physical process—light hitting retinas, signals traveling along optic nerves, brain regions activating. Perception is something else: the conscious experience of being aware of an object in the world. And here's Reid's key insight: the content of perception is not identical to the sum of sensations causing it.
When you look at a coffee cup, you have various sensations—patches of color, shapes, perhaps the warmth if you're holding it. But what you perceive is not a collection of color patches and thermal impressions. You perceive a coffee cup, an object that exists independently of you, that has a back side you can't currently see, that will still be there when you look away. The sensations are just the occasion for this perception, not its content.
Common Sense as a Technical Term
Reid borrowed the Latin term sensus communis from Cicero, but he gave it a specific philosophical meaning quite different from our everyday phrase "common sense" (meaning practical wisdom or good judgment).
For Reid, sensus communis refers to a set of foundational principles that all rational thought presupposes. These aren't conclusions we reason our way to—they're starting points that make reasoning possible in the first place.
Here's an example. When you have a philosophical argument with someone, you're implicitly assuming certain things: that the person you're talking to exists, that there's an external world governed by stable laws, that the words you're using refer to something, that your interlocutor can understand language, that the past happened roughly the way you remember it. If you tried to argue for these assumptions, you'd have to use the very faculties and principles you were trying to prove. The reasoning would be circular.
Reid's position was that these principles are not rational in the sense of being derived from reason. Rather, reason itself demands them as prerequisites. They're built into the "constitution" of the human mind. Denying them isn't just wrong—it's a kind of cognitive malfunction.
Reid put this rather pointedly. In Essays on the Intellectual Powers of Man, he wrote:
For, before men can reason together, they must agree in first principles; and it is impossible to reason with a man who has no principles in common with you.
Anyone who denies that qualities must belong to something, that a colored or shaped thing must exist to have those colors and shapes, is "not fit to be reasoned with." Reid wasn't being merely dismissive here. He was making a logical point: you can't argue someone into accepting the preconditions of argument. If they reject those, there's no common ground on which debate can proceed.
The Centaur Argument
Reid had a wonderfully clear way of making his case against the Theory of Ideas. Consider this: when you imagine a centaur, what are you imagining?
According to the Theory of Ideas, you're having an idea of a centaur—a mental representation. But Reid points out something peculiar about this. When I conceive of a centaur, the thing I'm conceiving is an animal (a mythological one, but still an animal). No idea is an animal. Therefore, what I'm conceiving is not an idea.
This argument works on two levels. First, there's the phenomenological point: when you introspect carefully on your experience of imagining a centaur, you're thinking about a creature, not about a mental state. The idea supposedly mediating your thought has become invisible, like a window so clean you don't notice the glass. Second, there's the linguistic point: when we use the word "centaur," we mean a half-human, half-horse creature, not a ghostly mental entity. The Theory of Ideas has smuggled in a technical meaning that doesn't match how we actually use language or experience thought.
Language and the Natural Signs
Reid had a fascinating theory about language that connects to his broader philosophy. He believed language rests on an innate capacity that predates human consciousness—something "natural" that enables the "artificial" system of conventional words and grammar we develop on top of it.
Think about how children learn to speak. They imitate sounds long before they understand meanings. If a baby needed to grasp abstract concepts before it could start imitating "mama" and "dada," it would never learn to talk at all. Reid concluded that there must be a layer of "natural signs"—gestures, tones, expressions—that carry meaning directly, without the arbitrary connection between sound and meaning that characterizes adult speech.
This distinction between natural and artificial signs matters for Reid's theory of perception too. In early human experience (and in children), perception has a quality of direct, unmediated contact with reality. We receive sensations "in their living nature," as Reid put it. Adults, focused on the practical business of identifying and manipulating objects, lose touch with this immediacy. We get so good at the artificial, conceptual overlay that we forget there's something more basic underneath.
Reid thought artists retained access to what he called "the language of nature"—they see colors and shapes as a child does, freshly, before the mind rushes to categorize and name. This isn't mysticism. It's an observation about attention: most of us, most of the time, pay attention to what things are (that's a chair, that's my friend, that's a threat) rather than how they appear in raw sensory experience.
Universal Agreement as Test of Reality
One of Reid's most interesting moves was his appeal to universal agreement. If all humans, observing the same thing, form the same beliefs about it, that's strong evidence those beliefs are true.
This isn't naive democratic epistemology—truth isn't determined by vote. Reid's point is different. The common sense principles he's defending aren't cultural conventions that vary from place to place. They're not things some societies believe and others reject. They're universal features of human cognition, present wherever you find functioning human minds. That universality suggests they're tracking something real about reality, not just artifacts of particular cultural training.
Reid compared this to the method of natural science as proposed by Francis Bacon. Scientists look for patterns that hold universally, under all conditions, regardless of who's observing. The "innate laws of nature" Bacon sought are just the regularities that every observer, everywhere, always finds. Reid thought the principles of common sense were similar: cognitive regularities that constitute the foundation for any possible investigation of the world.
The Aftermath: Influence and Eclipse
For several decades, Reid was considered the definitive answer to Hume. His Scottish School of Common Sense dominated philosophy in Scotland, spread through the British universities, and became the standard philosophical curriculum in American colleges throughout the 19th century. Victor Cousin championed Reid in France. When the Royal Society of Edinburgh was founded in 1783, Reid was one of its founding members.
Then came the eclipse. Immanuel Kant, writing in Germany, developed his own response to Hume—one that accepted the challenge of skepticism on different terms. Kant famously claimed never to have actually read Reid's work (while still criticizing it, which seems a bit unfair). John Stuart Mill attacked the Scottish School as well. By the late 19th century, Reid seemed like a historical curiosity, a sensible but philosophically shallow thinker who'd missed the real depth of the problems Hume raised.
But ideas have a way of returning. In the early 20th century, the Cambridge philosopher G.E. Moore developed a philosophical method based on appeals to common sense that bore striking resemblance to Reid's approach. More recently, Reid has attracted renewed attention from several quarters.
The American philosopher Charles Sanders Peirce, founder of pragmatism, acknowledged his debt to Reid—though Peirce's "critical common-sensism" modified Reid's ideas significantly. For Peirce, common sense is socially evolved and open to revision as evidence warrants, whereas Reid saw sensus communis as a fixed precondition for reasoning rather than its product.
Contemporary philosophers of religion, particularly those in the school of Reformed epistemology like Alvin Plantinga and William Alston, have found Reid useful for defending the rationality of religious belief. Their argument runs something like this: if Reid is right that certain basic beliefs are foundational and don't need to be derived from other beliefs, perhaps belief in God is one of these basic beliefs—rational not because it can be proven from other premises, but because it's properly foundational.
The Quiet Radicalism of Common Sense
There's something paradoxical about Reid's philosophy. It presents itself as humble, modest, the mere articulation of what everyone already knows. But it's actually a radical rejection of a centuries-long philosophical tradition. It says Descartes, Locke, Berkeley, and Hume all made the same fundamental error at the starting gate.
Reid's critics have always found this unsatisfying. Where's the argument? You can't just assert that common sense is correct and the great philosophers are wrong. What's the refutation of the Theory of Ideas?
But Reid would say that's missing the point. The Theory of Ideas isn't wrong because there's some clever argument against it—it's wrong because it leads to absurdity. If your philosophical premises imply that you can't know whether the external world exists, that's a reductio ad absurdum of those premises. The correct response is to go back and find where you went wrong, not to embrace skepticism about tables and chairs.
This connects to a broader point about philosophical method. Some philosophers think the goal is to start from the fewest possible assumptions and see what you can prove. Reid thought this was exactly backwards. We should start from what we know—the deliverances of common sense, confirmed by universal human experience—and use that to evaluate philosophical theories. Any theory that contradicts common sense is thereby refuted, not the other way around.
Whether you find this liberating or frustrating probably depends on your philosophical temperament. But it's worth taking seriously the possibility that Hume's skepticism, brilliant as it was, rested on a mistake in its very first move: the assumption that we're trapped behind a veil of ideas, never touching reality directly. Reid thought we touch reality all the time. Philosophy's job is to understand how, not to prove that we do.