PLN stands for Postfix Literal Notation (sometimes referred to as “reverse Polish notation” with reference to the Polish logician Jan Lukasiewicz, who substituted the letters of Polish words for the operators used by Russell and Whitehead). Its particular advantage is its adaptability to the computer, both in making use of an ordinary keyboard and in the relative ease of programming when it is the final letter of a string that expresses the operation in that string.

The general idea of postfix notation can be seen in expressing these arithmetic problems. In ordinary (infix) notation we have

2 + (3 x 4) to give us 14, or we have

(2 + 3) x 4 to give us 20. Without the parentheses it would be unclear just how the terms are connected.

In postfix notation the first problem becomes

2 3 4 x + (we multiply 3 and 4, then add to 2)

and the second becomes

2 3 + 4 x (we multiply 4 and the sum of 2 and 3).

In symbolic logic a conventional notation would show the conjunction of P and Q (saying both are true) as P & Q. In PLN we have PQA. Similarly we have (P&Q)->R for the idea that the conjunction of P and Q implies R. In PLN the same idea is expressed as PQARC.

Reading a PLN string carefully does take some practice. See the directions for the count for an easy technique to use as you get started.

Next: learn about variables and signals in PLN.

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