For Kant, the categorial principle of unity is a requirement for the very concept of nature. As he puts it in the Prolegomena to the Critique of Pure Reason, "nature is the existence of things, considered as existence determined according to universal laws." For Kant, the idea of God serves to symbolize or "schematize" the highest form of systematic unity to which empirical knowledge can be brought, the purposive unity of things. (B714)
According the Kant, the idea of space, a priori, is that of a unity. In the Critique of Pure Reason, Kant defined space as "the form of all phenomena of the external sense, that is, the subjective condition of the sensibility, under which alone external intuition is possible." (p.26) (For Ernst Cassirer, the recognition of non-Euclidean geometries seemed to mean renouncing the unity of reason, which is its intrinsic and distinguishing feature." (see scientific space )
In the Critique of Judgement, he realizes that the absolute conditions of experience are not enough, and that experience depends on our ability to arrange the particular laws of nature according to the idea of a system. The empirical unity of nature in all its diversity is not identical with the categorical one, not constitutive of our experience, but a regulative one.
It is the business of our understanding to introduce unity into nature. For a scientist to succeed at his task, he must assume that something corresponding to this unity actually exists in nature to be discovered.
The presupposition that things in nature are capable of generic classification is a particular case of this more general presupposition that nature is organized in such a way that empirical data can be brought into systematic unity. (the regulative concept functions "as if" it were the case. It is not necessarily actually so.) Since the faculty of judgement must aim at bringing about a system of particular laws but cannot know prior to actual experience whether it will able to do this or not, it is easy to understand why, according to Kant, we feel pleasure whenever we discover that two or more heterogeneous empirical laws have a common principle. Thus there is an a priori connection between the principle of judgement and the feeling of pleasure. In fact, observation itself depends on the principle that nothing about organized products of nature is gratuitous, that there is some principle of the unity of what is diverse, even though we do not know this principle.
Kant recognized two "interests" of reason. One of these had to do with unity and "homogeneity," (Geoffroy?) the other with multiplicity and "specification." (Cuvier?) According to Kant, controversies of this sort are incapable of being decided in such a way that one side is proved right or wrong. (see morphology) (see also analogy / homology )
From Aristotle and Alberti on, "Purposive unity" was the aspect of living nature that art should imitate above all others. (see organicism ) It was Plato in the Phaedrus who first enunciated the principle of organic unity in art. In it Socrates says that a composition "should be like a living being, with a body of its own as it were, and neither headless nor footless, but with a middle and members adapted to each other and the whole." Thus part of the classic formulation of organicism is that the parts of a work of art are not arbitrary or factitious, but as close and intimate as that between the organs of a living body. In his Poetics, Aristotle used the organic analogy to call for coherence of in time: "The construction of (epic) stories should be like that in a drama; they should be based on a single action, one that is a complete whole in itself, with a beginning, middle, and end, so as to enable the work to produce its own proper pleasure like a living creature which is one and a whole."
For theorists such as August Wilhelm von Schlegel, proportions and symmetry are signs of independence and wholeness in organic form in art and nature.
For Adolph Hildebrand, "the visual arts alone reflect the active operation of consciousness: the activity that seeks to bridge the gap between ideas of form and visual impressions and to fashion both into a unity." (Empathy, Form, and Space, p.231) "The true enjoyment of a work of art and its spontaneous blessing lies in the perception of this unity."
In Science and the Modern World, Alfred North Whitehead puts forward a doctrine of internal relations. He rejects the idea of simple location and says that reality is events (not things), which are prehensive unifications -- gathering diversities together in a unity; not simply here or there , but a gathering of here and there (subject and object) into a unity. Thus every event in nature is an organism which cannot be studied independent of the whole of which it is a part.
The bulk of Deleuze and Guattari's writing involves an attack on unity in the name of multiplicity. For D+G, " The notion of unity (unit ) "only appears when there is a power takeover in the multiplicity"