Simple example of necessity logic
B: Ensure a sufficient amount of heat
D: Buy a lighter
A: Start a campfire
| D-D': | Assumption(s) | Injection(s) |
| 1. I don't have money to buy both. 2. I don't already have either wood or a lighter lying around. 3. I can't borrow money. |
C: Have fuel
D': Buy wood
In order to Start a campfire I must Ensure a sufficient amount of heat and in order to Ensure a sufficient amount of heat I must Buy a lighter. But, in order to Start a campfire I must also Have fuel and in order to Have fuel I must Buy wood. I can't both Buy a lighter and Buy wood.
| Relation | Assumption(s) | Injection(s) |
| D-D' | 1. I don't have money to buy both. 2. I don't already have either wood or a lighter lying around. 3. I can't borrow money. | |
| B-D | 1. I don't have matches. 2. I don't have a magnifying glass and/or the sun isn't shining. 3. I can't borrow a lighter/matches from someone. | |
| C-D' | 1. I have nothing I can use as a fuel-source. 2. I can't get wood for free anywhere. | |
| A-B | 1. Heat is necessary to start a fire | |
| A-C | 1. Fuel is necessary to start a fire |