Full Idea
Many say everything is logically possible which involves no contradiction. In this sense two contradictory propositions may be severally possible. In the substantive sense, the contradictory of a possible proposition is impossible (if we were omniscient).
Gist of Idea
Some logical possibility concerns single propositions, but there is also compatibility between propositions
Source
Charles Sanders Peirce (The Nature of Mathematics [1898], III)
Book Reference
Peirce,Charles Sanders: 'Philosophical Writings of Peirce', ed/tr. Buchler,Justus [Dover 1940], p.146