Talk:Peirce's law

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\textbf{Peirce's law} is a proposition in propositional calculus that is commonly expressed in the following form:

\[ ((p \Rightarrow q) \Rightarrow p) \Rightarrow p \]

Peirce's law holds in classical propositional calculus, but not in intuitionistic propositional calculus.  The precise axiom system that one chooses for classical propositional calculus determines whether Peirce's law is taken as an axiom or proven as a theorem.

\section{History}

Here is Peirce's own statement and proof of the law:

\begin{quote}
A \textit{fifth icon} is required for the principle of excluded middle and other propositions connected with it.  One of the simplest formulae of this kind is:

\[ \{ (x \prec y) \prec x \} \prec x. \]

This is hardly axiomatical.  That it is true appears as follows.  It can only be false by the final consequent $x$ being false while its antecedent $(x \prec y) \prec x$ is true.  If this is true, either its consequent, $x$, is true, when the whole formula would be true, or its antecedent $x \prec y$ is false.  But in the last case the antecedent of $x \prec y$, that is $x$, must be true.  (Peirce, CP 3.384).
\end{quote}

Peirce goes on to point out an immediate application of the law:

\begin{quote}
From the formula just given, we at once get:

\[ \{ (x \prec y) \prec a \} \prec x, \]

where the $a$ is used in such a sense that $(x \prec y) \prec a$ means that from $(x \prec y)$ every proposition follows.  With that understanding, the formula states the principle of excluded middle, that from the falsity of the denial of $x$ follows the truth of $x$.  (Peirce, CP 3.384).
\end{quote}

\textbf{Note.}  The above transcription uses the ``precedes sign" ($\prec$) for the ``sign of illation'' that Peirce customarily wrote as a cursive symbol somewhat like a gamma ($\gamma$) turned on its side or else typed as a bigram consisting of a dash and a ``less than" sign.

\section{Graphic proof}

Under the existential interpretation of Peirce's logical graphs, Peirce's law is represented by means of the following formal equivalence or logical equation.

\begin{verbatim}
o-----------------------------------------------------------o
| Peirce's Law. . . . . . . . . . . . . . . . . . . . . . . |
o-----------------------------------------------------------o
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . p o---o q . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . o---o p . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . o---o p . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . @ . . . . . . . . = . . . . . . . . . @ . . . . |
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
o-----------------------------------------------------------o
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| . . .(((p (q)) (p)) (p))) . = . . . . . . . . . . . . . . |
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
o-----------------------------------------------------------o
\end{verbatim}

\textbf{Proof.}  Using the axiom set given in the entry for logical graphs, Peirce's law may be proved in the following manner.

\begin{verbatim}
o-----------------------------------------------------------o
| Peirce's Law. . Proof . . . . . . . . . . . . . . . . . . |
o-----------------------------------------------------------o
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . p o---o q . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . o---o p . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . o---o p . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . @ . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
o==================================< Collect >==============o
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . o---o q . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . o---o . . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . p o---o p . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . @ . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
o==================================< Recess >===============o
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . o---o . . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . p o---o p . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . @ . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
o==================================< Refold >===============o
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . p o---o p . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . @ . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
o==================================< Delete >===============o
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . o---o . . . . . . . . . . . . . . . . . . . . . |
| . . . . . | . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . @ . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
o==================================< Refold >===============o
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . @ . . . . . . . . . . . . . . . . . . . . . . . |
| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
o==================================< QED >==================o
\end{verbatim}

\section{Bibliography}

\begin{itemize}
\item
Peirce, Charles Sanders (1885), ``On the Algebra of Logic : A Contribution to the Philosophy of Notation", \textit{American Journal of Mathematics} 7 (1885), 180--202.  Reprinted (CP 3.359--403), (CE 5, 162--190).
\item
Peirce, Charles Sanders (1931--1935, 1958), \textit{Collected Papers of Charles Sanders Peirce}, vols. 1--6, Charles Hartshorne and Paul Weiss (eds.), vols. 7--8, Arthur W. Burks (ed.), Harvard University Press, Cambridge, MA.  Cited as (CP volume.paragraph).
\item
Peirce, Charles Sanders (1981--), \textit{Writings of Charles S. Peirce : A Chronological Edition}, Peirce Edition Project (eds.), Indiana University Press, Bloomington and Indianapolis, IN.  Cited as (CE volume, page).
\end{itemize}