Five questions.

  • What makes a fish a fish?
  • What makes a continent a continent?
  • What makes a country a country?
  • What makes a vegetable a vegetable?
  • What makes a sport a sport?

What makes a fish a fish?

  • Historically, animal groups (mammals, reptiles, birds, etc.) were defined by convention and common traits.
  • Biologists later started classifying animals by common ancestry.
  • Animals we call "fish" are very far apart evolutionarily.
  • A common ancestor would also include all mammals. That's a problem.

Let's ask Wikipedia.

A fish is an aquatic, anamniotic, gill-bearing vertebrate animal with a tough cranium to protect the brain, but lacking limbs with digits.

Included by the definition

Hagfish Seahorse Flying fish Mudskipper

Excluded by the definition

Whale Cuttlefish Jellyfish Starfish

What makes a continent a continent?

Let's ask Wikipedia.

[...] Continents are generally identified by convention rather than any strict criteria. [...]

Well, thanks for nothing...

Let's use a classroom definition.

Continents are large landmasses separated from others by oceans.
World map

Let's ask geologists.

Different continent models

What makes a country a country?

  • 193 UN member states
  • 7 additional partially recognized countries
  • 206 participating countries at the Olympics
  • Our definition needs to include the Vatican and exclude Hong Kong.

Let's look at common characteristics.

  • defined territory (or not)
  • permanent population (or not)
  • self-governing (or not)
  • international recognition (or not)
  • own currency (or not)
  • independent military (or not)

Some edge cases

Vatican Hong Kong Greenland Somaliland

What makes a vegetable a vegetable?

Let's ask Wikipedia.

Vegetables are edible parts of plants that are consumed by humans or other animals as food.

How wonderfully vague...

Why is it so hard to define?

  • Because it's not a botanical term, but a culinary one.
  • In botany we differentiate between seeds, roots, stems, flowers, fruits... and that's it. No vegetables.
  • So saying "a tomato is a fruit, not a vegetable" is a category error.

Is there a culinary definition?

  • In short: no.
  • Culinary classifications are not standardized and vary enormously between individuals and cultures.
  • The best "definition" I could find is: a plant we eat, but not for dessert.

Some edge cases

Mushroom Rhubarb Avocado Nori

What makes a sport a sport?

Let's ask Wikipedia.

Sport is a physical activity or game, often competitive and organized, that maintains or improves physical ability and skills.

That's not very helpful.

Let's look at common characteristics.

  • form of physical activity (or not)
  • competitive (or not)
  • done for its own sake (or not)
  • having clear rules (or not)
  • social (or not)

Some edge cases

Chess Darts Fishing Gaming

Five questions. One answer.

  • What makes a fish a fish? We do.
  • What makes a continent a continent? We do.
  • What makes a country a country? We do.
  • What makes a vegetable a vegetable? We do.
  • What makes a sport a sport? We do.

Categories compress information.

  • "Fish" saves you from listing every trait of every animal in the water.
  • "Vegetable" tells a cook roughly what to expect, in one word.
  • A category is a shared shortcut: we agree on it so we don't have to explain.

Compression is lossy.

  • A category hides differences between its members.
  • Hagfish and salmon are both "fish", yet they share less than salmon and humans do.
  • The edge cases are where the information gets lost.

When it stops compressing, discard it.

  • At the fishmonger, "fish" still works.
  • In evolutionary biology, it doesn't. Use clades instead.
  • Same word, different job. Keep it where it helps, drop it where it misleads.

Words are tools...

  • Tools should make life easier.
  • If they don't, they shouldn't be used.
  • Specific tools are made for specific situations.

Inspired by...