Full Idea
It is a common idea that demonstration must rest on indubitable propositions, either first principles of a general nature, or first sensations; but actual demonstration is completely satisfactory if it starts from propositions free from all actual doubt.
Gist of Idea
Demonstration does not rest on first principles of reason or sensation, but on freedom from actual doubt
Source
Charles Sanders Peirce (The Fixation of Belief [1877], p.11)
Book Reference
Peirce,Charles Sanders: 'Philosophical Writings of Peirce', ed/tr. Buchler,Justus [Dover 1940], p.11
A Reaction
Another nice example of Peirce focusing on the practical business of thinking, rather than abstract theory. I agree with this approach, that explanation and proof do not aim at perfection and indubitability, but at what satisfies a critical mind.