| Description: Define conjunction
(logical "and"). Definition of [Margaris] p. 49. When
both the left and right operand are true, the result is true; when either
is false, the result is false. For example, it is true that
(2 = 2 ∧ 3 = 3). After we define the constant
true ⊤
(df-tru 1543) and the constant false ⊥ (df-fal 1553), we will be able
to prove these truth table values: ((⊤ ∧
⊤) ↔ ⊤)
(truantru 1573), ((⊤ ∧ ⊥)
↔ ⊥) (truanfal 1574),
((⊥ ∧ ⊤) ↔ ⊥) (falantru 1575), and
((⊥ ∧ ⊥) ↔ ⊥) (falanfal 1576).
This is our first use of the biconditional connective in a definition; we
use the biconditional connective in place of the traditional
"<=def=>",
which means the same thing, except that we can manipulate the
biconditional connective directly in proofs rather than having to rely on
an informal definition substitution rule. Note that if we mechanically
substitute ¬ (𝜑 → ¬ 𝜓) for (𝜑 ∧ 𝜓), we end up with an
instance of previously proved theorem biid 261.
This is the justification
for the definition, along with the fact that it introduces a new symbol
∧. Contrast with ∨
(df-or 848), → (wi 4), ⊼
(df-nan 1492), and ⊻ (df-xor 1512). (Contributed by NM,
5-Jan-1993.) |
