The idea of nature is one of the most widely employed in philosophy, and by the same token one of the most ill-defined. Authors such as Aristotle and Descartes relied on the concept of nature to explain the fundamental tenets of their views, without ever attempting to define the concept. Even in contemporary philosophy, the idea is oftentimes employed, in different forms. So, what is nature?
Nature and the Essence of a Thing
The philosophical tradition that traces back to Aristotle employs the idea of nature to explain that which defines the essence of a thing. One of the most fundamental metaphysical concepts, the essence indicates those properties that define what a thing is. The essence of water, for instance, will be its molecular structure, the essence of a species, its ancestral history; the essence of a human, its self-consciousness or its soul. Within the Aristotelian traditions, hence, to act in accordance with nature means to take into account the real definition of each thing when dealing with it.
The Natural World
At times the idea of nature is instead used to refer to anything that exists in the universe as part of the physical world. In this sense, the idea embraces anything that falls under the study of the natural sciences, from physics to biology to environmental studies.
Natural vs. Artificial
"Natural" is often used also to refer to a process which occurs spontaneously as opposed to one that occurs as the result of the deliberation of a being. Thus, a plant grows naturally when its growth was not planned by a rational agent; it grows otherwise artificially. An apple, would hence be an artificial product, under this understanding of the idea of nature, although most would agree that an apple is a product of nature (that is, a part of the natural world, that which is studied by natural scientists).
Nature vs. Nurture
Related to the spontaneity vs. artificiality divide is the idea of nature as opposed to nurture. The idea of culture becomes here central to draw the line. That which is natural is opposed to that which is the outcome of a cultural process. Education is a central example of a non-natural process: under many accounts, education is seen as a process against nature. Clearly enough, from this perspective there are some items that cannot ever be purely natural: any human development is shaped by the activity, or lack thereof, of interaction with other human beings; there is no such thing as a natural development of human language, for instance.
Nature as Wilderness
The idea of nature is at times used to express wilderness. Wilderness lives at the edge of civilization, of any cultural processes. In the strictest reading of the term, humans can encounter wilderness in very few selected places on earth nowadays, those were the influence of human societies is negligible; if you include the environmental impact produced by humans on the whole ecosystem, there may well be no wild place left on our planet. If the idea of wilderness is loosened a bit, then even through a walk in a forest or a trip on the ocean one may experience that which is wild, i.e. natural.
Nature and God
Finally, an entry on nature cannot omit that which perhaps has been the most widely employed understanding of the term in past millennia: nature as the expression of the divine. The idea of nature is central in most religions. It has taken numerous forms, from specific entities or processes (a mountain, the sun, the ocean, or fire) to embracing the whole realm of existents.
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