[FOM] clean tautologies

[Neil Tennant]

Clean tautologies look to me like the perfect validities of my paper 
'Perfect validity, entailment and paraconsistency', Studia Logica XLIII, 
1984, pp. 179-198.

The notion of perfect validity was there defined for sequents, but
tautologies would come out as the special case where the antecedent of the
sequent is empty.

A sequent P1,...,Pn:Q is valid just in case no interpretation of the
language makes P1,...,Pn true but makes Q false. 

A sequent is perfectly valid just in case it is valid, has no valid proper
subsequent, and is not a proper substitution instance of any valid
sequent.

Neil Tennant