The second form of causality is the _stimulus_; it reigns over _organic_ life, as such, _i.e._ over plant life and the vegetative, that is, the unconscious, part of animal life. This second form is characterized by the absence of the distinctive signs of the first. In it accordingly action and reaction are not equal, nor does the intensity of the effect by any means correspond throughout all its degrees to the intensity of the cause; in fact, the opposite effect may even be produced by intensifying the cause. The third form of causality is the _motive_. Under this form causality rules animal life proper: that is, the exterior, consciously performed actions of all animals. The medium for motives is _knowledge_: an intellect is accordingly needed for susceptibility to motives. The true characteristic of the animal is therefore the faculty of knowing, of representing (_Das Vorstellen_). Animals, as such, always move towards some aim and end, which therefore must have been _recognised_ by them: that is to say, it must have presented itself to them as something different from themselves, yet of which they are conscious. Therefore the proper definition of the animal would be: 'That which knows;' for no other definition quite hits the mark or can even perhaps stand the test of investigation. Movement induced by motives is necessarily wanting where there is no cognitive faculty, and movement by stimuli alone remains, _i.e._ plant life. Irritability and sensibility are therefore inseparable. Still motives evidently act in a different way from stimuli; for the action of the former may be very brief, nay, need only be momentary; since their efficacy, unlike that of stimuli, stands in no relation whatever to the duration of that action, to the proximity of the object, &c. &c. A motive needs but to be perceived therefore, to take effect; whereas stimuli always require outward, often even inward, contact and invariably a certain length of time.