Full Idea
For a demonstration to be cogent it is necessary that the passage from step to step involve a recognition of truth at each line.
Gist of Idea
A successful proof requires recognition of truth at every step
Source
Michael Dummett (The Justification of Deduction [1973], p.313)
Book Reference
Dummett,Michael: 'Truth and Other Enigmas' [Duckworth 1978], p.313
A Reaction
Dummett cited Quine (esp. 1970) as having an almost entirely syntactic view of logic. Rumfitt points out that logic can move validly from one falsehood to another. Even a 'proof' might detour into falsehood, but it would not be a 'canonical' proof!