You probably feel certain about many things.
You know where you are. You know what you had for breakfast. You know that the Earth orbits the Sun, that Paris is in France and that other people have minds much like your own.
But these are surprisingly different kinds of knowledge.
You may remember breakfast yourself. You probably know about the Earth’s orbit through science and education. You may have visited Paris, or perhaps you simply trust maps and other people. And you cannot directly experience another person’s mind at all.
So what does it actually mean to know something?
How can we distinguish knowledge from belief, assumption or opinion?
These questions belong to epistemology, the branch of philosophy concerned with knowledge.
And they lead to an uncomfortable possibility:
How much of what we think we know could be wrong?
Can We Trust Our Senses?
Most of our knowledge of the world begins with our senses.
You see a tree and believe there is a tree in front of you. You hear thunder and know that a storm is nearby. You touch a cup and discover that it is hot.
This works remarkably well.
But our senses are not perfect.
A straight stick partly submerged in water can appear bent. Railway tracks seem to converge in the distance. Optical illusions can make stationary objects appear to move.
More importantly, our senses do not simply record reality like a camera.
The brain receives signals from our eyes, ears and other sensory organs and constructs an experience from them. What we consciously perceive is therefore already the result of considerable processing.
That does not mean our senses are useless or that the world is an illusion.
It means that perception gives us an interpretation of reality rather than completely unfiltered access to it.
This connects epistemology immediately with another philosophical question: What is reality actually like beyond our experience of it?
Could You Be Dreaming?
René Descartes took doubt much further.
He noticed that dreams can sometimes seem completely real while we are experiencing them.
Suppose you are sitting beside a fire reading a book. You see the flames, feel the warmth and hear the crackling wood.
How could you prove, with absolute certainty, that you are not dreaming?
Usually we recognise dreams because we eventually wake up. But while dreaming, we can temporarily accept extraordinary situations as real.
Descartes therefore decided to doubt anything that could possibly be doubted.
Perhaps his senses were deceiving him.
Perhaps he was dreaming.
He even imagined a powerful deceiver capable of manipulating every experience he had.
Centuries later, the same problem appears in thought experiments involving brains connected to computers and simulated realities.
Descartes was not claiming that we actually live inside an illusion. He was searching for something that remained certain even under the most extreme doubt.
He eventually found one thing.
I Think, Therefore I Am
Even if Descartes were being deceived about everything, he reasoned, something must exist to experience that deception.
He could doubt the world.
He could doubt his body.
He could even doubt his memories.
But he could not doubt that thinking was occurring while he was doubting.
This produced his famous conclusion:
“I think, therefore I am.”
The argument provided Descartes with a starting point, but it also reveals something peculiar about knowledge.
Your own conscious experience appears to be available to you in a way that the external world is not.
You can doubt whether the object you see really exists.
It is much harder to doubt that you are having the experience of seeing it.
But knowing that experience exists does not automatically tell us what lies beyond it.
Does Knowledge Come From Experience?
One of the great debates in philosophy concerns where knowledge comes from.
Empiricists emphasise experience.
John Locke argued that the mind does not begin life already filled with ideas about the world. Knowledge develops through experience.
David Hume pushed empiricism further. If an idea cannot ultimately be traced back to experience, he thought we should be suspicious of it.
This approach fits naturally with modern science.
We investigate the world by observing it, measuring it, conducting experiments and comparing results.
But experience alone creates difficulties.
No matter how many white swans you observe, you cannot logically prove from those observations alone that all swans are white. The next one might be black.
More generally, how do we know that patterns observed in the past will continue into the future?
We expect the Sun to rise tomorrow because it always has.
But what justifies the assumption that nature will continue behaving as it has before?
Hume showed that even ordinary expectations contain assumptions that are difficult to prove by experience alone.
Can Reason Give Us Knowledge?
Other philosophers have emphasised reason.
Some truths seem knowable without going out and examining the world.
You do not need to measure thousands of triangles to discover that certain conclusions follow from their geometry. Mathematical reasoning can establish relationships independently of any particular physical example.
Philosophers such as Descartes and Gottfried Leibniz therefore believed that reason could provide important knowledge.
This tradition became known as rationalism.
But reason has limitations too.
A perfectly logical argument can produce a false conclusion if it begins with false assumptions.
Reason may show us what follows from particular premises, but we still need to ask whether those premises accurately describe reality.
The tension between reason and experience became one of the central problems of philosophy.
Kant’s Attempt to Bring Them Together
Immanuel Kant believed that both sides had recognised something important.
Knowledge begins with experience, he argued, but does not arise entirely from experience.
The mind itself contributes something.
Imagine perception as information arriving from the world. Kant’s insight was that the mind does not simply receive that information passively. It organises experience into forms and categories that allow us to make sense of it.
We therefore do not encounter reality from a completely neutral position.
We encounter it through the structure of the human mind.
This helped Kant explain how knowledge could depend upon both the world and the way our minds organise experience.
But it also created a boundary.
We may know reality as it appears to beings like us without knowing reality exactly as it exists independently of our way of experiencing it.
Once again, the question What can we know? leads back to What is reality?
What Counts as Knowledge?
Philosophers have traditionally distinguished knowledge from simply having a belief that happens to be correct.
Suppose you guess that it will rain tomorrow and, by chance, it does.
Your belief was true.
But did you know it would rain?
Probably not.
Knowledge seems to require something more — some justification or reliable basis for believing what is true.
For centuries philosophers often discussed knowledge in terms of justified true belief:
You believe something.
It is true.
And you have good reasons for believing it.
That seems plausible, but twentieth-century philosopher Edmund Gettier showed that even these conditions can produce strange cases where someone reaches a true belief through a mixture of justification and luck.
The resulting Gettier problem helped demonstrate how surprisingly difficult it is to define knowledge precisely.
Fortunately, everyday life does not require us to solve the Gettier problem before saying that we know where we left the car.
But it reveals that the word know contains more complexity than we normally notice.
How Do We Know What Other People Know?
There is another important source of knowledge that philosophy can easily overlook:
other people.
You probably have not personally measured the distance from Earth to the Sun.
You have not repeated every experiment supporting atomic theory.
You did not witness most historical events you believe occurred.
Much of what any individual knows depends upon testimony — information received from scientists, historians, teachers, journalists, books, institutions and other people.
That is not a weakness. Human civilisation depends upon shared knowledge.
No individual could independently verify everything.
The difficult question is deciding whom to trust, about what, and why.
Expertise matters. Evidence matters. Independent confirmation matters. The methods used to reach a conclusion matter.
This has become particularly important in a world where almost anyone can publish information instantly and where false claims can spread as easily as reliable ones.
Learning how knowledge is produced may therefore be as important as learning the knowledge itself.
How Certain Do We Need to Be?
Perhaps one mistake is assuming that knowledge requires absolute certainty.
Outside mathematics and logic, much of what we reasonably call knowledge comes in degrees of confidence.
Science rarely proves claims with the kind of certainty Descartes was seeking. Instead, explanations are tested against evidence.
A theory becomes more credible when it successfully explains observations, makes accurate predictions and survives attempts to prove it wrong.
New evidence can still force scientists to revise even successful theories.
That willingness to revise is not a weakness of science.
It is one of its strengths.
The same principle can be useful more generally.
Instead of dividing every belief into certain or false, we can ask:
How strong is the evidence?
How reliable is the source?
What alternative explanations are possible?
What evidence would make me change my mind?
These are philosophical questions, but they are also practical tools for thinking.
Could We Know Nothing at All?
The most extreme position is skepticism.
Perhaps we cannot know anything about the external world with complete certainty.
We could be dreaming. Our memories could be mistaken. Our senses could deceive us. A sufficiently advanced simulation could theoretically produce experiences indistinguishable from ordinary reality.
But radical skepticism creates its own problem.
Even if absolute certainty is impossible, some beliefs are clearly much better supported than others.
There is an enormous difference between believing something because repeated observation and experiment support it and believing something simply because we would like it to be true.
Philosophical skepticism therefore does not have to mean refusing to believe anything.
Used carefully, it can mean asking how we know what we think we know.
That habit lies at the heart of philosophy, science and critical thinking.
Knowledge Requires Humility
One of philosophy’s most useful lessons may be recognising the limits of our own certainty.
Human beings are capable of extraordinary discoveries, but we are also prone to confirmation bias, faulty memory, wishful thinking and overconfidence.
We naturally notice evidence supporting what we already believe.
We can remember events incorrectly while feeling certain that our memories are accurate.
Groups can reinforce beliefs simply because everyone around them appears to agree.
None of this means that truth is impossible.
It means that finding truth requires methods designed to correct our individual weaknesses: evidence, reason, criticism, debate, replication and a willingness to change our minds.
There is a considerable difference between saying “nothing can be known” and saying “I might be wrong.”
The second may actually help us know more.
So What Can We Know?
There is no universally accepted philosophical boundary separating everything we can know from everything we cannot.
Our senses provide enormously useful information but do not give us completely unfiltered access to reality.
Reason allows us to draw conclusions but depends upon assumptions and premises.
Memory is indispensable but imperfect.
Other people provide most of the knowledge no individual could discover alone, but sources differ greatly in reliability.
Science gives us perhaps the most successful method yet developed for understanding the physical world, while remaining open to revision when better evidence appears.
Philosophy helps us examine the assumptions underlying all these methods.
Perhaps the most sensible position is neither complete certainty nor complete skepticism.
We can have well-supported beliefs while remaining willing to revise them.
Knowledge need not mean possessing an infallible view of reality.
It can mean having good reasons for believing something while understanding the limits of those reasons.
Where the Question Leads
Once we ask what we can know, other Great Questions quickly appear.
If all our knowledge of the external world comes through perception, what is reality actually like beyond our experience of it?
If our brains construct our experience of the world, what is the relationship between mind and body?
If our thoughts and decisions arise from processes we do not fully understand, do we have free will?
And behind all of them lies an even stranger fact:
we are conscious beings trying to understand a universe from inside the universe itself.
Perhaps we will never possess complete knowledge of reality.
But recognising what we know, what we merely believe, why we believe it and what might prove us wrong is already a powerful beginning.
The question “How do you know?” may be one of the simplest questions in philosophy.
It is also one of the most important.