Full Idea
It is so important, if we are to have a clear insight into what is going on, for us to be able to recognise the premises of every inference which occurs in a proof and the law of inference in accordance with which it takes place.
Gist of Idea
We must be clear about every premise and every law used in a proof
Source
Gottlob Frege (Logic in Mathematics [1914], p.212)
Book Reference
Frege,Gottlob: 'Posthumous Writings', ed/tr. Hermes/Long/White etc [Blackwell 1979], p.212
A Reaction
Teachers of logic like natural deduction, because it reduces everything to a few clear laws, which can be stated at each step.