remembering that many feelings which do resemble each other closely,--e. g., toothaches--do not on that account know each other. Really to know a thing, a feeling must not only resemble the thing, but must also be able to act on it. In brief, "the feeling of _q_ knows whatever reality it resembles, and either directly or indirectly operates on. If it resemble without operating, it is a dream; if it operates without resembling, it is an error". Such is the formula for perceptual knowledge. Concepts must be reduced to percepts, after which the same rule holds. We may say, to make the formula complete, "A percept knows whatever reality it directly or indirectly operates on and resembles; a conceptual feeling, or thought, knows a reality, whenever it actually or potentially terminates in a percept that operates on, or resembles that reality, or is otherwise connected with it or with its context". "The latter percept [the one to which the concept has been reduced] may be either sensation or sensorial idea; and when I say the thought must _terminate_ in such a percept, I mean that it must ultimately be capable of leading up thereto,--by way of practical experience if the terminal feeling be a sensation; by way of logical or habitual suggestion, if it be only an image in the mind". "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 the other, and the reduction of the substitute to the status of a conceptual sign. Condemned 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 meanings.... We can never be sure we understand each other till we are able to bring the matter to this test. 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".