What can save us at all and prevent us from flying asunder into a chaos of mutually repellent solipsisms? Through what can our several minds commune? Through nothing but the mutual resemblance of those of our perceptual feelings which have this power of modifying one another, WHICH ARE MERE DUMB KNOWLEDGES-OF-ACQUAINTANCE, and which must also resemble their realities or not know them aright at all. In such pieces of knowledge-of-acquaintance all our knowledge-about must end, and carry a sense of this possible termination as part of its content. These percepts, these termini, these sensible things, these mere matters-of-acquaintance, are the only realities we ever directly know, and the whole history of our thought is the history of our substitution of one of them for another, and the reduction of the substitute to the status of a conceptual sign. Contemned though they be by some thinkers, these sensations are the mother-earth, the anchorage, the stable rock, the first and last limits, the terminus a quo and the terminus ad quem of the mind. To find such sensational termini should be our aim with all our higher thought. They end discussion; they destroy the false conceit of knowledge; and without them we are all at sea with each other's meaning. If two men act alike on a percept, they believe themselves to feel alike about it; if not, they may suspect they know it in differing ways. We can never be sure we understand each other till we are able to bring the matter to this test. [Footnote: 'There is no distinction of meaning so fine as to consist in anything but a possible difference of practice.... It appears, then, that the rule for attaining the [highest] grade of clearness of apprehension is as follows: Consider what effects, which might conceivably have practical bearings, we conceive the object of our conception to have. Then, our conception of these effects is the whole of our conception of the object.' Charles S. Peirce: 'How to make our Ideas clear,' in Popular Science Monthly, New York, January, 1878, p. 293.] This is why metaphysical discussions are so much like fighting with the air; they have no practical issue of a sensational kind. 'Scientific' theories, on the other hand, always terminate in definite percepts. You can deduce a possible sensation from your theory and, taking me into your laboratory, prove that your theory is true of my world by giving me the sensation then and there. Beautiful is the flight of conceptual reason through the upper air of truth. No wonder philosophers are dazzled by it still, and no wonder they look with some disdain at the low earth of feeling from which the goddess launched herself aloft. But woe to her if she return not home to its acquaintance; Nirgends haften dann die unsicheren Sohlen--every crazy wind will take her, and, like a fire-balloon at night, she will go out among the stars.