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Description: Define the biconditional
(logical 'iff').
The definition dfbi 185 in this section is our first definition, which
introduces and defines the biconditional connective Unlike most traditional developments, we have chosen not to have a separate symbol such as "Df." to mean "is defined as." Instead, we will later use the biconditional connective for this purpose (dfor 370 is its first use), as it allows us to use logic to manipulate definitions directly. This greatly simplifies many proofs since it eliminates the need for a separate mechanism for introducing and eliminating definitions. Of course, we cannot use this mechanism to define the biconditional itself, since it hasn't been introduced yet. Instead, we use a more general form of definition, described as follows. In its most general form, a definition is simply an assertion that introduces a new symbol (or a new combination of existing symbols, as in df3an 975) that is eliminable and does not strengthen the existing language. The latter requirement means that the set of provable statements not containing the new symbol (or new combination) should remain exactly the same after the definition is introduced. Our definition of the biconditional may look unusual compared to most definitions, but it strictly satisfies these requirements. The justification for our definition is that if we mechanically replace (the definiendum i.e. the thing being defined) with (the definiens i.e. the defining expression) in the definition, the definition becomes the previously proved theorem bijust 183. It is impossible to use dfbi 185 to prove any statement expressed in the original language that can't be proved from the original axioms, because if we simply replace each instance of dfbi 185 in the proof with the corresponding bijust 183 instance, we will end up with a proof from the original axioms. Note that from Metamath's point of view, a definition is just another axiom  i.e. an assertion we claim to be true  but from our high level point of view, we are not strengthening the language. To indicate this fact, we prefix definition labels with "df" instead of "ax". (This prefixing is an informal convention that means nothing to the Metamath proof verifier; it is just a naming convention for human readability.) After we define the constant true (dftru 1398) and the constant false (dffal 1401), we will be able to prove these truth table values: (trubitru 1433), (trubifal 1434), (falbitru 1435), and (falbifal 1436).
See dfbi1 192, dfbi2 628, and dfbi3 893
for theorems suggesting typical
textbook definitions of
Contrast with 
Ref  Expression 

dfbi 
Step  Hyp  Ref  Expression 

1  wph  . . . . 5  
2  wps  . . . . 5  
3  1, 2  wb 184  . . . 4 
4  1, 2  wi 4  . . . . . 6 
5  2, 1  wi 4  . . . . . . 7 
6  5  wn 3  . . . . . 6 
7  4, 6  wi 4  . . . . 5 
8  7  wn 3  . . . 4 
9  3, 8  wi 4  . . 3 
10  8, 3  wi 4  . . . 4 
11  10  wn 3  . . 3 
12  9, 11  wi 4  . 2 
13  12  wn 3  1 
Colors of variables: wff setvar class 
This definition is referenced by: bi1 186 bi3 187 dfbi1 192 dfbi1ALT 193 
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