Each of the following arguments in English may be similarly translated, and for each, a formal proof of validity (using only the nine elementary valid argument forms as rules of inference) may be constructed. These proofs vary in length, some requiring a sequence of thirteen statements (including the premises) to complete the formal proofs. The suggested abbreviations should be used for the sake of clarity. Bear in mind that, as one proceeds to produce a formal proof of an argument presented in a natural language, it is of the utmost importance that the translation into symbolic notation of the statements appearing discursively in the argument be perfectly accurate ; if it is not, one will be working with an argument that is different from the original one, and in that case any proof devised will be useless, being not applicable to the original argument.

If Brown received the message, then she took the plane; and if she took the plane, then she will not be late for the meeting. If the message was incorrectly addressed, then Brown will be late for the meeting. Either Brown received the message or the message was incorrectly addressed. Therefore either Brown took the plane or she will be late for the meeting. ( R —Brown received the message; P —Brown took the plane; L —Brown will be late for the meeting; T —The message was incorrectly addressed.)