The Commens Dictionary
Quote from ‘Issues of Pragmaticism’
Restricting the word to its characteristic applicability, a state of things has the Modality of the possible – that is, of the merely possible – only in case the contradictory state of things is likewise possible, which proves possibility to be the vague modality. One who knows that Harvard University has an office in State Street, Boston, and has impression that it is at No. 30, but yet suspects that 50 is the number, would say “I think it is at No. 30, but it may be at No. 50,” or “it is possibly at No. 50.” Thereupon, another, who does not doubt his recollection, might chime in, “It actually is at No. 50,” or simply “it is at No. 50,” or “it is at No. 50, de inesse.” Thereupon, the person who had first asked, what the number was might say, “Since you are so positive, it must be at No. 50,” for “I know the first figure is 5. So, since you are both certain the second is a 0, why 50 it necessarily is.” That is to say, in this most subjective kind of Modality, that which is known by direct recollection is in the Mode of Actuality, the determinate mode. But when knowledge is indeterminate among alternatives, either there is one state of things which alone accords with them all, when this is in the Mode of Necessity, or there is more than one state of things that no knowledge excludes, when each of these is in the Mode of Possibility.