2528. Now, the animal is distinguished only from Nature by the act of being liberated from it. It is therefore comprehended in a constant liberation or severance. The life of the animal continues only by a constantly renewed and indefatigable severance, by a desertion or falling off from Nature.
2529. In so doing, however, it detaches itself from Nature, as being a part thereof; the severance is therefore a conversion of the nature into an animal. In this consists the reciprocal action of both, viz. that the animal is constantly seeking to assimilate the nature unto itself.
2530. The ability or power to assimilate the nature, is called excitability.
2531. Excitability is the most general phenomenon of the organic mass, and appertains both to plants and animals.
2532. But in the animal excitability, the free self-sensation, within which a free motion is necessarily inherent, is superadded or originates. This excitability unto motion I call irritability.
2533. Thus this irritability belongs only to animals.
2534. Irritability does not depend directly upon motion, but throughout upon sensation. Without sensation no irritability is possible. If the sensation ceases, so also does mobility, or the capacity for motion, cease.
2535. Since irritability originates from the antagonism of the animal with the world; so is it parallel to an antagonism of the heavenly bodies, or to that of sun and planets. The mutual operation of these two heavenly bodies is, however, an interchange of polarity, a polar excitation. The irritability is a polar process; but one which is pure and devoid of material excretions, just as the sun excites the earth without eliciting therein any material change or transition. The animal becomes polarized by the incentive agent or stimulus.
2536. Through the irritability there originates a double polarity in the animal. In the first place one between the world and the animal; in the second, one between the exterior of the animal and its interior. The world-polarity gives the feeling or sensation, the body's polarity the motion.
2537. In the sensation the animal always transcends itself; there is thus only excitability. In the motion the animal abides or remains within itself; but the self-sensation proceeds from both conditions. Accordingly, in self-sensation the world, and the animal within the animal, convene or come together. The animal is itself universe, and it at the same time comprehends the great universe. Now, in both conditions or in both functions has the animal been turned towards the world and also towards itself. In feeling it turns itself towards the world, in order to adopt this spiritually into, or repel this from, itself; in motion it turns itself towards the world, in order to materially adopt or to repel it. In both cases of assumption it turns itself towards itself.