The unity of the theoretical and practical idea leads into an immediacy. Here, what is given overlaps totally with what has become. Here is where the "absolute idea" comes on the scene. The "absolute idea" is the name for the point where logic transitions into nature.
What is Hegel's definition of nature?
The idea, namely, in positing itself as the absolute unity of the pure concept and its reality and thus collecting itself in the immediacy of being, is in this form a totality -- nature. (12.253)
Nature is here defined as the idea as (1) immediate, yet (2) totality. The idea remains self-identical totality even when it "freely discharges" itself in nature.
On account of this freedom, the form of its determinateness is just as absolutely free: the externality of space and time absolutely existing for itself without subjectivity. (12.253)
The definition here put forth can be clarified with reference to Schelling's concept of nature in his First Outline for a Philosophy of Nature and Collingwood's An Essay on Philosophical Method. For Schelling, the problem to be solved with the philosophy of nature is not how movement or change is caused in nature, but rather why there is a product at all. The unconditioned is as such totality for Schelling, and there is no necessity in this totality to crystallize into a "product," this finite, perishable natural thing. Hegel agrees with Schelling on this point. For both, nature is a product, a representation of a certain failure of the absolute idea to fully become itself. The question becomes how and in what sense the absolute idea distinguishes nature, or space and time, from itself. If the absolute idea is the totality of all logical determinations, how could nature escape this totality? How could a category be posited by the absolute idea yet elude its grasp?
In order to answer this question, I now turn to Collingwood. For Collingwood, in a philosophical concept, the species of a genus overlap in the genus' realization. For example, if the category of the "whole" is the truth of the category of the "part," this does not merely mean that whole and part are two distinct categories, but it also means that the whole is the unity of whole and part, and vice versa. This overlapping is not an arbitrary feature introduced by Collingwood's caprice. It is rather deeply engrained in the very thing called thought, and is deduced rigorously by the opening chapters on being and nothing in the Science of Logic.
In the absolute idea, the logic returns to its beginning, but the return tries to leave behind a totality of all categories. However, in so far as in pure logic all categories overlap with each other, the beginning as such already contains the totality of categories. Therefore, according to the logic's own logic, or according to the very fundamental feature of thought that is deduced from the first beginning of logic, logic cannot simply return to its first beginning. Nature is posited as the totality of categories which do not overlap. Natural categories are posited in space and time, are unified in space and time, and as such do not overlap, are wholly external to each other. It is in this sense that nature is free from pure logic. Yet the categorical determinations in nature are "discharged" from the absolute idea's failure to return to its beginning. It is this failure that links pure logic to nature and necessitates the transition from the former to the latter.
As with all other key claims made in the Science of Logic, the transition to nature is not an ad hoc inference which is supported merely by the historical fact that in 19th century Germany there was such a thing as the idea of nature. Rather, nature is here deduced, and the meaning of the term "nature" is strictly defined and determined as the field of pure externality where the absolute idea "freely discharges" its logical determinations or categories.
Some Hegel scholars have argued that the Science of Logic already implicitly contains all the "logical structures" of nature and spirit. On this interpretation, these scholars have tried to demonstrate how the Science of Logic already has a certain "spatial" or "temporal" structure. I think that this line of interpretation is wrong in light of the above considerations on the definition of nature. The "structure" of nature is that it is a wholly new beginning which only emerges as a result of the absolute idea or pure logic failing to negate the feature of overlap amongst its categories. In one sense, since these categories are nonetheless "discharged," it is partly true to claim that pure logic makes some contribution to the organization of nature. Yet spatial and temporal order is not determined or organized at all in pure logic, precisely because space and time are the first marks of nature's absolute independence from the overlapping character of the categories of pure logic. Moreover, the aforementioned interpretation also misses the much less subtle point that, if pure logic can by itself deduce spatial and temporal structures within itself, then there is no necessity for it to posit nature or spirit - and if there is no necessity, there is no freedom, according to the familiar formula.
There is much more to be said about this solution to the final riddle in the Science of Logic, but for now, this is good enough.
