Game table — ca. 1780–83
Mathematics
·
Nothing Is Certain — But You Can Get Close
Easy
5 min
Nothing Is Certain — But You Can Get Close
How to stop guessing blindly and start thinking in chances
Share
You're about to leave the house. No umbrella. You glance outside — cloudy, but not raining. Do you risk it? Whatever you just did in your head — that gut calculation about likelihood — that's probability. And you can get dramatically better at it.
Here's what probability actually is: a number between 0 and 1 that tells you how likely something is to happen. Zero means impossible. One means certain. Everything interesting lives in between. A 0.5 probability means something happens about half the time. A 0.1 means one time in ten. A 0.99 means it almost always happens — but 'almost' is doing real work in that sentence. You already use this every day. You just don't write the numbers down.
Analogy
Think about your group chat. You don't know exactly who'll reply first when you drop a message — but you have a sense. If Maya answers within a minute every single time and Jake surfaces once a week, you're not expecting Jake. You've built a mental model of likelihood based on patterns. That's exactly how probability works. You look at what's happened before, or what's possible, and you assign weights to outcomes. Some things are more likely. Some are less. Probability is just the tool that makes those weights precise instead of fuzzy.
The cleanest way to calculate a probability: count the outcomes you care about, divide by the total number of possible outcomes. Flip a coin. Two possible outcomes — heads or tails. You care about heads. So: 1 out of 2, which is 0.5, which is 50%. Roll a six-sided die. You want a 4. One outcome you care about, six total outcomes. Probability = 1/6, roughly 16.7%. This only works when every outcome is equally likely — which isn't always true. But it's the foundation everything else builds on.
Key Insight
Probability doesn't tell you what WILL happen. It tells you what to EXPECT on average, over many tries. A 10% chance doesn't mean it won't happen to you first. It means if a thousand people faced the same situation, about 100 of them would get that outcome. You might be one of them. You might not. That uncertainty is the whole point.
Make your call
Back to that umbrella decision: if you apply to five internships each accepting 1 in 10 applicants, what are the odds you walk away with nothing? Trust your gut first.
A
Somewhere around 10–20% — five tries makes it pretty likely you land one
B
About 50% — basically a coin flip across five attempts
C
Around 59% — you're more likely to get rejected by all five than your gut suggests
Example
Say you're applying to five selective internships, each of which accepts roughly 1 in 10 applicants. Your gut says: five tries, surely I'll get at least one? Here's where probability gets humbling. The chance of getting rejected by any single internship is 0.9 (90%). The chance of getting rejected by all five? You multiply: 0.9 × 0.9 × 0.9 × 0.9 × 0.9 = about 0.59. That's a 59% chance of getting nothing. Still more likely than not to get at least one offer — but nowhere near the certainty your gut assumed. This is why thinking in numbers beats thinking in vibes.
Now it gets interesting — probability is also about information. The same event can have a different probability depending on what you know. You draw a card from a deck. Before you look: 1 in 52 chance it's the ace of spades. After you look at it and see it's a spade: now there's a 1 in 13 chance it's the ace of spades. The card didn't change. Your information did. And that changed the probability. This is why weather forecasts update throughout the day. New data comes in, the probabilities shift. The storm doesn't move — the knowledge does.
Key Insight
The most powerful thing probability gives you isn't certainty — it's calibration. Knowing the difference between a 5% chance and a 40% chance means you make better decisions, take smarter risks, and stop being surprised by things that were always pretty likely. Most people never learn this. You now have a head start — including on that cloudy-morning umbrella call, which just got a little more precise.
Related threads
Mathematics
·
Why Half of Something Isn't Always Equal
Why Half of Something Isn't Always Equal
Sources
Images from Wikimedia Commons. Attribution required by license.
Game table — ca. 1780–83
David Roentgen via The Metropolitan Museum of Art (Public Domain)
Scope and content: The full caption for this photograph reads: Heart Mountain Relocation Center, Heart Mountain, Wyoming. In his barracks home at Block 7 - 21, Bill Hosokawa and his wife Alice serves oyster stew in an evening's visit with the members of the War Relocation Authority appointed personnel. Left to right is Julona Steinheider, High School Mathematics Teacher; Margaret Jensen, Center Librarian; Vaughn Mechau, Reports Officer; Bill, his son Mike and his wife, Alice. (This appears to have been written from a flipped image, but there's plenty of signs - the hands and watches being appropriate for right-handed people, and the calendar in the background, while not legible, looking more appropriate as it is now than in a mirror flip, that indicates this arrangement is correct).
Unknown authorUnknown author or not provided · Public domain
Quiz yourself