The Commens Dictionary
Quote from ‘The Regenerated Logic’
…possibility may be understood in many senses; but they may all be embraced under the definition that that is possible which, in a certain state of information, is not known to be false. By varying the supposed state of information all the varieties of possibility are obtained. Thus, essential possibility is that which supposes nothing to be known except logical rules. Substantive possibility, on the other hand, supposes a state of omniscience.
‘Substantial Possibility’ (pub. 11.08.17-10:19). Quote in M. Bergman & S. Paavola (Eds.), The Commens Dictionary: Peirce's Terms in His Own Words. New Edition. Retrieved from http://www.commens.org/dictionary/entry/quote-regenerated-logic-8.