Natural deduction in the forall x: Mississippi State systems
This document gives a short description of how Carnap presents the systems of natural deduction from Greg Johnson's forall x: Mississippi State. At least some prior familiarity with Fitch-style proof systems is assumed.
The syntax of formulas accepted is described in the Systems Reference.
The different admissible keyboard abbreviations for the different connectives are as follows:
The available sentence letters are A through Z, together with the
infinitely many subscripted letters P1, P2, … written
P_1, P_2 and so on.
Proofs consist of a series of lines. A line is either an assertion
line containing a formula followed by a
: and then a justification
for that formula, or a separator line containing two dashes, thus:
--. A justification consists of a rule abbreviation followed by zero
or more numbers (citations of particular lines) and pairs of numbers
separated by a dash (citations of a subproof contained within the
given line range).
A subproof is begun by increasing the indentation level. The first line of a subproof should be more indented than the containing proof, and the lines directly contained in this subproof should maintain this indentation level. (Lines indirectly contained, by being part of a sub-sub-proof, will need to be indented more deeply.) The subproof ends when the indentation level of the containing proof is resumed; hence, two contiguous sub-proofs of the same containing proof can be distinguished from one another by inserting a separator line between them at the same level of indentation as the containing proof. The final unindented line of a derivation will serve as the conclusion of the entire derivation.
Here's an example derivation, using the TFL system
Or, with a Fitch-style guides overlay (activated with
There is also a playground mode:
The system for Johnson's forall x: Mississippi State (the system used in
a proofchecker constructed with
.JohnsonSL in Carnap's Pandoc
Markup) has the following set of rules for direct
|φ ∧ ψ
|φ ∧ ψ
|¬ψ, φ ∨ ψ
|¬φ, φ ∨ ψ
|φ ∨ ψ
|φ ∨ ψ
|φ, φ → ψ
|φ, φ ↔︎ ψ
|ψ, φ ↔︎ ψ
|φ → ψ, ψ → φ
|φ ↔︎ ψ
We also have four rules for indirect inferences:
→I, which justifies an assertion of the form φ → ψ by citing a subproof beginning with the assumption φ and ending with the conclusion ψ;
¬I, which justifies an assertion of the form ¬φ by citing a subproof beginning with the assumption φ and ending with a pair of lines ψ,¬ψ.
¬E, which justifies an assertion of the form φ by citing a subproof beginning with the assumption ¬φ and ending with a pair of lines ψ,¬ψ.
PR can be used to justify a line asserting a premise, and
AS can be used to justify a line making an assumption. A note about
the reason for an assumption can be included in the rendered proof by
A/NOTETEXTHERE rather than
AS for an assumption.
Assumptions are only allowed on the first line of a subproof.