How Do We Know What We Know? Al-Fārābī on the Mind, the Universe, and Certainty

Imagine you’re sitting in class and your teacher says, “The whole is greater than the part.” You don’t need to think about it. You just know it’s true. But how? Where did that knowledge come from? You didn’t learn it from experience—you’ve never seen every possible whole and part. You didn’t figure it out by reasoning step by step. It’s just there, solid and obvious, as if your mind already had it waiting.

This is the kind of puzzle that fascinated Abū Naṣr al-Fārābī, a philosopher who lived in Baghdad over a thousand years ago. He wanted to understand how human beings go from being born with empty minds to having knowledge—not just facts, but deep understanding of things that are always and necessarily true. His answers took him on a strange journey through the structure of the universe, the nature of the soul, dreams, prophecy, and what it really means to be certain of something. And along the way, he ended up with a picture of the human mind that is still surprising today.

The Universe as a Chain of Intellects

To understand how al-Fārābī thought about knowledge, you first have to understand how he thought about everything else. He believed the universe was arranged like a giant chain, with each link giving existence to the one below it.

At the very top is something he called the First Existent. This is not a person-like God who makes decisions or gets angry. It is pure intellect—a perfect mind that thinks only about itself. Because it is completely perfect and simple, it cannot decide to create anything. Instead, everything else necessarily flows out of it, the way light necessarily flows from the sun. Al-Fārābī called this process “emanation.”

From the First Existent came a series of ten intellects, each one thinking about the First Existent and about itself. These intellects are not physical things—they are pure thought. And as they think, they produce the physical universe. The first nine intellects are responsible for the celestial spheres: the stars, the planets, the moon. They don’t just make these bodies exist; they keep them moving in their perfect circular orbits.

The tenth intellect is special. Al-Fārābī called it the Active Intellect. Its job is to govern everything below the moon—our world of plants, animals, minerals, and humans. And here’s the crucial part: the Active Intellect is also what makes human knowledge possible. Without it, we would be stuck with nothing but our senses and our imagination, unable to reach any real understanding.

The Soul and Its Faculties

Al-Fārābī believed that every living thing has a soul, but souls come in different levels depending on what the living thing can do. Plants have a soul that handles nutrition and reproduction. Animals have that plus sensation, appetite (desire and aversion), and imagination. Humans have all of that plus something extra: a rational faculty that can think about abstract truths.

The rational faculty itself has two parts. The practical intellect deals with things we can change—like figuring out how to build a shelf, deciding whether to tell the truth, or calculating the best way to get somewhere. The theoretical intellect deals with things that are always true and can’t be changed—like mathematics, the laws of logic, and the nature of existence itself.

But here’s the puzzle. When you’re born, you don’t already know these truths. You have to discover them. Yet some truths—like “the whole is greater than the part”—seem to be there in your mind before any experience teaches them to you. How can that be?

The Imagination: More Powerful Than You’d Think

Before we get to how knowledge works, we need to talk about something al-Fārābī thought was surprisingly important: the imagination.

You probably think of imagination as making stuff up—daydreaming, pretending, creating stories. That’s part of it. But al-Fārābī saw imagination as a kind of middle-man between your senses and your reason. When you see a tree, your senses give you an impression. When you close your eyes, your imagination holds onto that impression. It can also combine impressions: you’ve never seen a purple zebra, but your imagination can make one.

But imagination can do something even stranger. Al-Fārābī thought it could imitate things that aren’t sensory at all. For example, if you have a deep abstract thought about justice, your imagination might represent it as a picture of a balanced scale or a person holding a sword. The imagination is trying to make something non-physical feel physical.

This becomes really important when al-Fārābī talks about prophecy. He believed that some people—very few—have such powerful imaginations that they can receive knowledge directly from the Active Intellect without going through their senses first. This knowledge comes to them in the form of vivid images, dreams, or visions. They see what is true, but they see it in pictures rather than in abstract concepts.

This was al-Fārābī’s way of explaining how prophets get their knowledge without contradicting his view that real understanding comes through reason. The prophet gets the same truths that a philosopher gets by thinking, but the prophet gets them through imagination. This made some people angry, because it seemed to reduce prophecy to a kind of natural brainpower rather than a miracle. But al-Fārābī didn’t think he was demoting prophecy—he thought he was explaining how it actually works.

The Intellect: From Empty to Full

Now we come to the heart of the matter. How does the human mind actually know things?

Al-Fārābī, following Aristotle, described the intellect as going through stages. Imagine a blank piece of wax. That’s the material intellect—the raw potential to think, which every human has from birth. It’s not thinking anything yet. It just can think.

Now imagine something pressing into the wax and leaving an impression. That’s what happens when you learn something. But here’s the catch: the things you learn from your senses—like “this apple is red”—are tied to particular times and places. They’re not universal truths. To get universal truths, something more has to happen.

When you learn, for example, that three is an odd number, your mind has to abstract the form of “threeness” away from any particular group of three things. You’ve never seen “three” itself. You’ve only seen three apples, three pencils, three chairs. But somehow your mind can grasp what “three” is on its own, separate from any specific stuff.

How does that happen? According to al-Fārābī, it happens because the Active Intellect—that tenth intellect from the chain of the universe—acts like a light. Just as sunlight makes colors visible to your eyes, the Active Intellect illuminates the forms hidden in physical things so that your material intellect can grasp them.

This is a genuinely strange idea. It means that, for al-Fārābī, your mind doesn’t just learn from experience. Experience provides the raw material, but the understanding comes from something outside you—something cosmic. The Active Intellect is like a transmitter, and your mind is like a receiver. When you finally grasp a universal truth, your mind becomes one with that truth. At that moment, the knower and the known are the same thing.

As you keep learning, your intellect moves through stages:

Material intellect — just the potential to think
Actual intellect — when you are actually thinking about something you’ve learned
Acquired intellect — when your mind has become so developed that it can think on its own, without needing the senses anymore

The acquired intellect is the most advanced stage. At this point, al-Fārābī believed, the human intellect becomes so perfect that it no longer needs the body. It can exist on its own, like the celestial intellects. This, for him, was what happiness really meant: not pleasure or wealth, but becoming an independent thinking substance that continues to exist after death.

Certainty: Knowing That You Know

Al-Fārābī was very strict about what counts as real knowledge. He didn’t think most of what people call “knowledge” deserves the name.

For something to be true knowledge, he said, it has to meet several conditions. You have to believe it. It has to actually be true. You have to know that it’s true—not just guess. It has to be impossible for it to be false. And it has to be impossible for it to ever become false, at any time. Finally, all of these conditions have to hold essentially, not just by accident.

That’s a lot. Most of what we say we “know” doesn’t pass this test. I “know” the sun will rise tomorrow, but it’s not impossible that the sun could explode tonight. I “know” my name, but someone could have changed the records. Genuine certainty, for al-Fārābī, is extremely rare and extremely valuable.

This kind of knowledge comes only through demonstration—logical arguments that start from absolutely certain premises and move step by step to absolutely certain conclusions. Other forms of reasoning—dialectic (arguing from common opinions), rhetoric (persuasion by eloquence), poetics (persuasion by images)—can produce belief, sometimes very strong belief. But they don’t produce certainty.

Al-Fārābī thought that this was why philosophy was so important. Only philosophical demonstration could give you the real thing: knowledge that couldn’t possibly be wrong, that you knew you couldn’t be wrong about, and that would last forever.

Still Alive

Al-Fārābī’s system is elegant and strange. It ties together the structure of the cosmos, the biology of the human body, the workings of the imagination, and the nature of knowledge into one unified picture. But it also raises questions that philosophers still argue about.

Is knowledge really that rare? Do you need something like the Active Intellect to explain how humans understand universals? Can the mind survive without the body? And what about truths that seem certain but turn out to be wrong—like people once being “certain” that the sun went around the Earth?

Al-Fārābī wouldn’t have been bothered by that last question, because he would say those people weren’t truly certain—they were just strongly convinced. Real certainty, for him, must be impossible to be wrong about. But that raises another question: does anything at all meet that standard? Even math? Even logic?

Nobody really knows. But that’s exactly the kind of question al-Fārābī would have wanted you to ask.

Appendices

Key Terms

Term	What it does in this debate
Active Intellect	The tenth and lowest celestial intellect that illuminates the human mind, making abstract knowledge possible
Material intellect	The raw potential to think that every human is born with—like a blank slate
Actual intellect	The mind when it is actively thinking about something it has learned
Acquired intellect	The highest stage of human thinking, where the mind can understand on its own without needing the body
Emanation	The process by which everything in the universe flows necessarily from the First Existent, like light from the sun
Demonstration	A type of logical argument that starts from certain premises and produces absolute certainty
Imaginative faculty	The part of the soul that stores sense impressions, combines them, and can imitate abstract truths as images
Certainty	Knowledge that is true, known to be true, impossible to be false, and essentially so

Key People

Al-Fārābī (c. 870–950 CE): A philosopher from Central Asia who worked in Baghdad and became known as the “Second Teacher” (after Aristotle). He tried to bring together Greek philosophy with Islamic thought and wrote about nearly everything: logic, politics, music, psychology, and the nature of the universe.
Aristotle (384–322 BCE): The Greek philosopher whose ideas about the soul, the intellect, and scientific knowledge were the foundation for al-Fārābī’s entire project. Al-Fārābī saw himself as continuing and completing Aristotle’s work.
Alexander of Aphrodisias (c. 200 CE): A Greek commentator on Aristotle whose ideas about the material and active intellect were borrowed and developed by al-Fārābī.

Things to Think About

Al-Fārābī thought that real certainty requires you to know that you know—to be aware of your own knowledge. But if you have to check whether you’re certain, doesn’t that mean you weren’t certain in the first place? Can you be certain without knowing that you’re certain?
If all human knowledge depends on the Active Intellect shining its light on our minds, does that mean people in different times and places see different truths? Or is the Active Intellect the same for everyone, making truth universal?
Al-Fārābī thought the imagination could receive truth directly from the Active Intellect in the form of images and dreams. How would you tell the difference between a genuine prophetic vision and a dream that’s just your brain mixing up random memories?
If the highest form of knowledge is about things that can never change, what about knowledge of how to live, how to treat other people, or how to build a just society? Are those things changeable, or are there universal truths about them too?

Where This Shows Up

In debates about artificial intelligence: When programmers try to figure out whether a computer can “really” think or just process information, they’re wrestling with the same question al-Fārābī asked about the difference between having data and having understanding.
In psychology and neuroscience: Researchers who study how the brain learns abstract concepts (like numbers or justice) are trying to explain something al-Fārābī already knew was mysterious: how do we get from particular experiences to universal ideas?
In arguments about education: The idea that some knowledge is “innate” or that the teacher’s job is to “illuminate” what’s already there rather than stuffing facts in—that’s al-Fārābī’s view showing up in modern classrooms.
In discussions about certainty and belief: When people talk about “fake news” or “alternative facts,” they’re dealing with al-Fārābī’s distinction between what we’re convinced of and what we genuinely know. He thought those were very different things, and he was right.