Wood is not enough

About this video

Ana has collected a pile of dry wood on a camping trip. "We've got wood, so we can have a fire!" But her dad left the matches at home. Wood is needed for a fire: without something to burn, there can be no fire at all. But a fire also needs heat to start it and air to keep it going. Take away any one of the three and the fire goes out. Something can be needed and still not be enough.

Something is enough for a result when, as soon as you have it, the result is sure. Being a square is enough for having four sides. But it isn't needed: a rectangle and a kite have four sides too. Logicians call what is needed a necessary condition and what is enough a sufficient condition.

This animation ends with the two tests. Is it needed? Try to imagine the result without it. Is it enough? Try to imagine having it without the result. Mixing the two up is one of the most common mistakes in reasoning: "I studied, so I'll pass" treats something needed as if it were enough. This video goes with the Needed or Enough? lesson of Logic Level 2 on Knaph.

Transcript

1. 0:00We've got wood, so we can have a fire! Can we?
2. 0:05A fire needs fuel, heat and air. Take any one away, and it goes out. Each one is needed.
3. 0:12But wood alone is not enough. No match, no fire.
4. 0:17A square always has four sides, so it's enough. But a rectangle has four sides too. Not needed.
5. 0:24Needed? Imagine the result without it. Enough? Imagine it without the result.
6. 0:29Needed is not the same as enough.