The separation of an emotional element in our integral reaction to a situation is more or less artificial. No doubt there is a definite physiological concomitant, namely stimulation of a gland; but fear, for example, involves a mode of action towards an object, for which mode of action the secretion of adrenin is helpful. There is, however, something in common among a number of occasions that have a given emotional tone; this may be seen from the fact that they are associated. When we are feeling some emotion strongly, we tend to think of other occasions when we have had similar feelings. Association by means of emotional similarity is a characteristic of a great deal of poetry. And this accounts for the fact that, if our blood is in a state usually associated with terror, we shall, if our critical faculty is in abeyance, be very likely to imagine some cause of fear so vividly as to believe that it is really present: In the night, imagining some fear, How easy is a bush supposed a bear. But in a rational man, if he is not drunk or sleepy, other associations are too strong for this production of imaginary terrors. That is why it is possible to show the physical symptoms of fear under the influence of adrenin, without actually feeling the emotion. The emotions are what makes life interesting, and what makes us feel it important. From this point of view, they are the most valuable element in human existence. But when, as in philosophy, we are trying to understand the world, they appear rather as a hindrance. They generate irrational opinions, since emotional associations seldom correspond with collocations in the external world. They cause us to view the universe in the mirror of our moods, as now bright, now dim, according to the state of the mirror. With the sole exception of curiosity, the emotions are on the whole a hindrance to the intellectual life, though the degree of vigour required for successful thinking is likely to be correlated with a considerable susceptibility to emotion. If I say little about the emotions in this book, it is not from under-estimating their human importance, but solely because the task upon which we are engaged is theoretical rather than practical: to understand the world, not to change it. And if emotion determines the ends we shall pursue, knowledge is what gives us the power to realise them. Even from the practical point of view, the advancement of knowledge is more useful than anything else that lies within human power. I come now to the subject of _desire_, which we considered from a behaviourist standpoint in Chapter III. I want now to ask whether there is anything to be added from an introspective point of view.