In Nature, as in Mind, the most typical phenomenon is an original heterogeneity, duplicity, or difference, which, however, points back to a still more fundamental homogeneity, unity, or identity. This primary unity or ground of unification does not indeed appear to sight; the 'soul of Nature,' the _anima mundi,_ nowhere presents itself as such in its undivided simplicity; but only as the perpetually recurring re-union of what has been divided. But though unapparent, the absolute identity is the necessary presupposition of all life and existence, as of all knowledge and action. It is the link or 'copula' which perpetually reduces the antithesis to unity, and the heterogeneity to homogeneity, and the different to redintegration. To this fact of antithesis, presupposing and continually reverting to an original unity, Schelling gives the name 'Polarity,' 'It is impossible to construe the main physical phenomena without such a conflict of opposite principles. But this conflict only exists at the instant of the phenomenon itself. Each natural force _awakes_ its opposite. But that force has no independent existence: it only exists in this contest, and it is only this contest which gives it for the moment a separate existence. As soon as this contest ceases, the force vanishes, by retreating into the sphere of homogeneous forces[2].' Polarity, therefore, is a general law of the cosmos. A ceaseless, limitless activity, therefore, as the basis or groundwork of all, for ever crossing, arresting, and limiting itself: an eternal war, which, however, is always being led back to peace,--a process of differentiation which rests upon, is the product of, and is for ever forced back to integration, is the perpetual rhythm of the natural universe. It is a process in which can be traced three grades, stages, or 'powers' (first, second, and third, &c.). By its more generally descriptive name it is called Organisation. 'Organism,' says Schelling, 'is the principle of things. It is not a property of single natural objects; but, on the contrary, single natural objects are so many limitations, or single modes of apprehending the universal organism[3].' 'The world is an organisation; and a universal organism itself is the condition (and to that extent the positive) of mechanism[4].' 'Mechanism is to be explained from organism: not organism from mechanism.' 'The _essential_ of all things is _life_: the _accidental_ is only the _kind_ of their life: and even the dead in Nature is not utterly dead,--it is only extinct life.'