Formal Logical Possibility
…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.
Very many writers assert that everything is logically possible which involves no contradiction. Let us call that sort of logical possibility, essential, or formal, logical possibility. It is not the only logical possibility; for in this sense, two propositions contradictory of one another may both be severally possible, although their combination is not possible.