The key to this difficulty lies in the distinction between knowing as verified and completed, and the same knowing as in transit and on its way. To recur to the Memorial Hall example lately used, it is only when our idea of the Hall has actually terminated in the percept that we know 'for certain' that from the beginning it was truly cognitive of _that_. Until established by the end of the process, its quality of knowing that, or indeed of knowing anything, could still be doubted; and yet the knowing really was there, as the result now shows. We were _virtual_ knowers of the Hall long before we were certified to have been its actual knowers, by the percept's retroactive validating power. Just so we are 'mortal' all the time, by reason of the virtuality of the inevitable event which will make us so when it shall have come. Now the immensely greater part of all our knowing never gets beyond this virtual stage. It never is completed or nailed down. I speak not merely of our ideas of imperceptibles like ether-waves or dissociated 'ions,' or of 'ejects' like the contents of our neighbors' minds; I speak also of ideas which we might verify if we would take the trouble, but which we hold for true although unterminated perceptually, because nothing says 'no' to us, and there is no contradicting truth in sight. _To continue thinking unchallenged is, ninety-nine times out of a hundred, our practical substitute for knowing in the completed sense._ As each experience runs by cognitive transition into the next one, and we nowhere feel a collision with what we elsewhere count as truth or fact, we commit ourselves to the current as if the port were sure. We live, as it were, upon the front edge of an advancing wave-crest, and our sense of a determinate direction in falling forward is all we cover of the future of our path. It is as if a differential quotient should be conscious and treat itself as an adequate substitute for a traced-out curve. Our experience, _inter alia_, is of variations of rate and of direction, and lives in these transitions more than in the journey's end. The experiences of tendency are sufficient to act upon--what more could we have _done_ at those moments even if the later verification comes complete?