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Full Idea
For mathematics, Weyl arrived (by 1917) at a satisfactory list of definition principles: negation, identification of variables, conjunction, disjunction, substitution of constants, and existential quantification over the domain.
Gist of Idea
Definition just needs negation, known variables, conjunction, disjunction, substitution and quantification
Source
report of Hermann Weyl (works [1917]) by Shaughan Lavine - Understanding the Infinite V.3
Book Ref
Lavine,Shaughan: 'Understanding the Infinite' [Harvard 1994], p.125
A Reaction
Lavine summarises this as 'first-order logic with parameters'.