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#### 6.1.1 Modus Ponens

- P -> Q
- P
- ——–
- Q

One of the basic rules of logic, modus ponens say that ‘`if P happens, then Q must happen. P happened, so Q must happen`’.

For example, if it is known that ‘`If the dog begins to bark, then someone is at the door`’, and it is also known that ‘`the dog has begun to bark`’, then modus ponens says that ‘`someone must be at the door`’.

Modus Ponens requires exactly two references.