Desire is extremely subject to “conditioning”. If A is a primitive desire and B has on many occasions been a means to A, B comes to be desired in the same sense in which A was previously desired. It may even happen, as in misers, that the desire for B completely displaces the desire for A, so that B, when attained, is no longer used as a means to A. This, however, is more or less exceptional. In general, the desire for A persists, although the desire for B has a more or less independent life. The “conditioning” of primitive desires in human beings is the source of much that distinguishes our life from that of animals. Most animals only seek food when they are hungry; they may, then, die of starvation before they find it. Men, on the contrary, must have early acquired pleasure in hunting as an art, and must have set out on hunting expeditions before they were actually hungry. A further stage in the conditioning of hunger came with domestic animals; a still further stage with agriculture. Nowadays, when a man sets to work to earn his living, his activity is still connected, though not very directly, with hunger and the other primitive desires that can be gratified by means of money. These primitive desires are still, so to speak, the power station, though their energy is widely distributed to all sorts of undertakings that seem, at first sight, to have no connection with them. Consider “freedom” and the political activities it inspires; this is derivable, by “conditioning”, from the emotion of rage which Dr. Watson observed in infants whose limbs are not “free”. Again we speak of the “fall” of empires and of “fallen” women; this is connected with the fear which infants display when left without support. After this excursion into the realm of desire, we can now return to “knowledge”, which, as we saw, is a term correlative to “desire”, and applicable to another feature of the same kind of activity. We may say, broadly, that a response to a stimulus of the kind involving desire in the above sense shows “knowledge” if it leads by the quickest or easiest route to the state of affairs which, in the above sense, is behaviouristically the object of desire. Knowledge is thus a matter of degree: the rat, during its progressive improvements in the maze, is gradually acquiring more and more knowledge. Its “intelligence quotient”, so far as that particular task is concerned, will be the ratio of the time it took on the first trial to the time it takes now to get out of the maze. Another point, if our definition of knowledge is accepted, is, that there is no such thing as purely contemplative knowledge: knowledge exists only in relation to the satisfaction of desire, or, as we say, in the capacity to choose the right means to achieve our ends.