Full Idea
What is missing, above all, in 'Principia', is a precise statement of the syntax of the formalism.
Gist of Idea
'Principia' lacks a precise statement of the syntax
Source
comment on B Russell/AN Whitehead (Principia Mathematica [1913]) by Kurt Gödel - Russell's Mathematical Logic p.448
Book Reference
'Philosophy of Mathematics: readings (2nd)', ed/tr. Benacerraf/Putnam [CUP 1983], p.448