Can You Trust Your Senses? A Thinker's Answer
The Illusion at the Market
You’re walking through a busy market, sunlight bouncing off everything. A flash of silver catches your eye, resting on a merchant’s cloth. You reach out, already picturing the ring it could become. But when your fingers close around it, the object feels wrong—light, cool, maybe a bit rough. It’s not silver at all. It’s a piece of mother‑of‑pearl, a shell.
Your senses told you one thing; reality said another. So how can you trust what you see, hear, or reason? That’s exactly the kind of puzzle that fascinated Gaṅgeśa (fourteenth century), a brilliant philosopher who lived in Mithilā, in what is now northeastern India. He became one of the most important thinkers in the history of Indian logic and epistemology—the study of knowledge.
Gaṅgeśa wrote a book nicknamed the Jewel, and its central question is straightforward: How do we know anything, and how can we be sure our knowledge is right? His answer changed the way people thought about perception, reasoning, and even what it means to be fooled.
Four Keys to Knowing
Gaṅgeśa argued that we don’t rely on just one way of finding things out. He identified four independent knowledge sources, each with its own trigger that sets it in motion.
The first is perception (pratyakṣa). When your eyes connect with a clay pot, or your skin feels the warmth of the sun, a knowing‑event is born. The trigger is the contact between a sense organ and an object. Perception feels immediate and natural—so natural that we usually trust it without a second thought.
Second comes inference (anumāna). Imagine you see smoke curling above a distant hill. You don’t see the fire, but you know it’s there. How? Because you’ve learned a rule: wherever there’s smoke, there’s fire. Inference depends on grasping this kind of universal connection, which Gaṅgeśa called pervasion. The trigger is a mental act of reflecting on that connection and seeing that it applies right now.
The third source is analogy (upamāna). Suppose you hear the word “gavaya” (a kind of wild buffalo), but you have no idea what it refers to. Someone says, “A gavaya is like a cow, but wilder and darker.” Later, in the forest, you spot just such an animal and think, This is what that word means! Analogy gives you the meaning of a word by connecting a description to a real thing. Its trigger is the remembered similarity.
Finally, there’s testimony (śabda). When a trustworthy person—a teacher, a doctor, a reliable friend—tells you something true, you gain knowledge without seeing or inferring it yourself. The trigger is the hearer’s comprehension of the speaker’s statement, provided the speaker knows the truth and is not trying to deceive. For Gaṅgeśa, language itself, when used honestly, is a genuine path to knowledge.
Checking the Checkers: Certification
If these sources are so reliable, how do we ever get things wrong? And how can we tell when we do? Gaṅgeśa’s answer is that an awareness—the “fresh news” your mind receives from any of the four sources—is always taken to be true at first. You don’t normally doubt your own eyes. But that confidence is defeasible: it can be overturned later by a defeater.
Maybe you remember that the speaker is a known liar. Or you realize the light was too dim to see clearly. In such cases, the awareness turns out not to be genuine perception, inference, analogy, or testimony. It was a pseudo‑source, a look‑alike process that produced a mistake.
To separate real knowledge from illusion, Gaṅgeśa said we need a second‑level act of certification. Often this works by inference: “If that had not been real silver, I wouldn’t have been able to sell it for a good price. But I did sell it, so my perception was probably correct.” Another way is to identify which specific knowledge source produced the awareness. If you can tell that your awareness came from perception, and you can check that the conditions were good (enough light, healthy eyes, no trickery), the presumption of truth gets stronger.
This “extrinsic” view set Gaṅgeśa against other thinkers who believed knowledge is self‑certifying—that we always know immediately that we know, without any extra step. Gaṅgeśa pointed out that doubt happens all the time. If knowledge came with its own built‑in guarantee, you’d never be unsure whether the shiny thing was silver. But you are unsure, and that very experience proves that confirmation is needed.
Smoke, Fire, and the Sneaky Extra Condition
Of all the ways we know, inference was Gaṅgeśa’s special love. He devoted a huge part of the Jewel to making the rules of reasoning airtight. At the heart of any inference lies a pervasion: “Wherever there is H, there is S.” H is the prover (like smoke), and S is the probandum (like fire).
But what if the rule has a hidden loophole? Gaṅgeśa gave a famous example: suppose you infer, “This tree has a monkey in it, because it is this tree.” You know this particular tree always seems to have monkeys. Yet that rule only works for the branches, not the roots! A property like “monkey‑conjunction” is not present in every part of the tree—so a good inference has to account for that complexity.
Even worse is the sneaky upādhi, an “additional condition” that can poison an inference without you noticing. Imagine you conclude, “Al will lose his temper now,” because you’ve seen him explode many times. But what if Al only ever explodes when a certain person—Belle—is in the room, and she left five minutes ago? Belle’s presence is the upādhi. If you miss it, your conclusion is unsound. Gaṅgeśa analyzed upādhis and many other fallacies with painstaking care, turning informal hunches into a system of logical defenses.
He also argued that even when an inference is well‑supported, absolute certainty is rare. A clever opponent could always imagine a counterexample—like discovering diamonds, which are made of earth yet cannot be scratched by iron, overturning an old rule. For Gaṅgeśa, that was fine. Knowledge doesn’t require a perfect, doubt‑free guarantee. It requires enough positive evidence and no active defeaters. You can still act, and you do.
Why a 700‑Year‑Old Argument Still Matters
Today, you probably don’t worry about wild buffaloes or sacrificial rituals. But you live in a world flooded with claims—social media posts, news headlines, advertisements, friend‑of‑a‑friend stories. Gaṅgeśa’s framework offers a surprisingly handy toolkit.
His first lesson is that there are several distinct ways good information can reach you: seeing with your own eyes, reasoning carefully, comparing a new thing to something known, and listening to people who actually know what they’re talking about. The second lesson is that none of these is magic. You’re allowed—in fact, encouraged—to double‑check. Does the source have a defeater? Can you certify it, even roughly? If not, your confidence should drop.
Gaṅgeśa didn’t invent critical thinking, but he built one of history’s most rigorous systems for it. His insistence that knowledge is fallible, yet still worth acting on, speaks directly to a question you face almost every day: Is this true enough to go with? The Indian logicians who followed him spent centuries unpacking his every sentence, a tribute to just how deep his questions were. And the next time you spot something shiny, hesitate, and take a better look, you’re right there with him in the market.
Think about it
- If you learn a fact only from a friend’s text message, would you say you “know” it? What would have to be true about the friend and the message for you to be sure?
- A clever used‑car salesperson gives you a long, logical‑sounding argument why a rusty car is a great buy. Later, a mechanic shows the engine is shot. Did the salesperson’s argument count as knowledge for you before you learned the truth?
- Could there ever be a situation where you should trust an inference even though you have not personally checked every step? Where would you draw the line?
