Any discussion of animal behaviour must deal largely with what is termed the conative aspect of consciousness. “The states designated by such words as _craving_, _longing_, _yearning_, _endeavour_, _effort_, _desire_, _wish_, and _will_,” says Dr. Stout, in his admirable “Manual of Psychology,”[117] “have one characteristic in common. In all of these there is an inherent tendency to pass beyond themselves and become something different. This tendency is not only a fact, but an experience; and the peculiar mode of being conscious, which constitutes the experience, is called _conation_.” Closely associated with this conation is _impulse_, which Dr. Stout defines as “any conative tendency, so far as it operates by its own isolated intensity, apart from its relation to a general system of motives. Action on impulse is thus contrasted with action which results from reflection and deliberation.”[118] In the interpretation we have advocated, animals are essentially creatures of impulse, and not to any large extent, if at all, reflective agents. And their impulses may be associated either with their inherited and congenital behaviour, or with that which is due to acquired experience. In other words, their impulses may be divided broadly into two classes, the one instinctive, the other acquired. Dr. Stout says that conation is not only a fact, but an experience. Now, first as to the fact. It seems to be the correlative in consciousness of the behaviour of the nervous system under stimulation. Let us take some simple case such as that, for example, of a hungry chick pecking at a grain of corn. This is explained from the physiological point of view by saying that internal and external stimulation--internal from the digestive organs and system in need of food, external through the eye--gives rise to a state of unstable equilibrium in the nerve-centres; and that, when the instability reaches a certain value, the nervous system discharges into the motor organs, the chick pecks, and stable equilibrium is restored. The tendency to discharge in some way under stimulation is an essential characteristic of a nervous system. It is one of the facts of physiological science. So, too, the conative tendency is one of the facts of psychological science; it is a change in the situation introduced by the effects of the physiological discharge.