The Second Law of Thermodynamics is stated in various ways, but its basic idea is that the total entropy or disorder in a closed system will always increase rather than decrease with time. This is true of the Universe as a whole, or of much smaller parts of it, such as what happens in a sealed room into which no energy of any kind is introduced or removed. A specific application of the Second Law of Thermodynamics is to the concept of perpetual motion machines: because of friction, which results in the transformation of kinetic energy into heat (disorder), these kinds of machines cannot exist. In other words, in order to keep a machine running, it requires a regular input of energy of some kind. Another way of thinking of the Second Law is that whenever energy is transformed from one form to another, there is always a net loss of usable energy, even though the total energy of the system remains constant, in accordance with the Law of the Conservation of Energy.
The following excerpt illustrates a fairly common misconception about the Second Law of Thermodynamics:
Scientists have long been baffled by the existence of spontaneous order in the universe. The laws of thermodynamics seem to dictate the opposite, that nature should inexorably degenerate toward a state of greater disorder, greater entropy. Yet all around us we see magnificent structures – galaxies, cells, ecosystems, human beings – that have somehow managed to assemble themselves. This enigma bedevils all of science today.
The author of this excerpt is not the only person who has misunderstood the true nature of scientific laws like the Second Law of Thermodynamics. This is clear from his use of the word “dictate”, which is usually employed to describe human actions and decisions, and not the interactions of inanimate matter. As the philosopher David Hume observed, scientific laws are only statements or generalizations about certain observed regularities in the world or Universe.
The answer to the riddle posed by the excerpt is simple: although the Universe as a whole tends towards greater entropy, there is no reason why there cannot exist pockets or smaller units of order within the greater scheme of generally increasing disorder. This is in fact what all life is – a constant struggle to preserve and perpetuate its particular form of order against the Universe’s tendency towards disorder, a fact that becomes apparent following death, when the formerly living body, no longer able to keep itself together by protecting and repairing itself, is subjected to the process of decomposition and disintegration into smaller and smaller particles.
If the Earth is able to support a vast abundance and variety of life, it is because it continually receives an enormous amount of energy every day from the Sun. Without this regular and prodigious input of energy, we would soon see the terrible effects of the Second Law of Thermodynamics – terrible at least from the perspective of all forms of terrestrial life. This solar energy is transformed, first of all, by plants into chemical energy, which, besides allowing them to grow and reproduce, is utilized by other organisms that feed on plants, which, in turn, support other forms of life. But at each stage of transformation, there is a significant net loss of useable energy, which accounts for the pyramidal structure of species, with plants at the base, tapering up while their numbers rapidly decrease, towards the apex predators. Even after death, all organisms continue to support some form of life, such as insects, bacteria, and fungi, which decompose the dead remains into a form that can be utilized by plants so they can continue the cycle of life and death. Although one may wonder at the mystery of the existence and variety of life, there is no mystery about how life on Earth is able to continue to exist and reproduce itself.
There is another mistake made by those who are puzzled by the Second Law of Thermodynamics and the seeming contradiction between order (life) and disorder (death or chaos): you cannot use a scientific law that is derived from the observation of actual events to argue that some of those events are improbable or impossible. Stated in this manner, it is obvious that this is a completely illogical way of proceeding. And yet, those writers and thinkers who are perplexed by the existence of order because of the Second Law of Thermodynamics commit precisely this error. They fail to understand that all scientific laws, which are, without exception, generalizations based on observed phenomena, must be consistent with all observed phenomena. If there is an inconsistency, whether it is real or apparent, between a scientific law or theory and reality, then either there is something wrong with the law or theory, or, as in this case, there is something wrong with one’s understanding of the law or theory.
But I suspect there is another reason for this fairly common error. As religious belief has declined in some countries, in large part because of the success of science in explaining phenomena that previously seemed miraculous or were incomprehensible to our ancestors, as well as the accompanying success of technology in shielding those of us who are fortunate enough to live in wealthy industrialized countries from the terrible ravages of Nature, there has yet remained a residue of wonder about the mystery of life. And science, despite its many successes in explaining to us life’s physical, chemical, genetic, and molecular processes, has not been able to account for the fact that life exists at all. In the past, these observations led people to worship a deity or deities that they believed had created them and all the other animals and organisms that exist, whose existence allowed them to survive. But now, those who believe in science are presented instead with impersonal laws and theories that have displaced the God or gods in which almost all people formerly believed. And it is precisely this wonder – at the fact that we and all other organisms should exist at all – that science will never be able to explain away, no matter how successful it is in reducing life to purely mechanical processes.
 Sync: The Emerging Science of Spontaneous Order by Steven Strogatz. Hachette Books, 2003.
 In fairness to the author, it appears that he also seems to understand this point when he writes, immediately following the quoted passage, “The key to unlocking the mysteries of self-organization, according to Belgian chemist Ilya Prigogine and his colleagues, lies in a deeper understanding of thermodynamics, where emergence of order is a victorious uphill battle against entropy, as a complex system feeds itself on energy flowing in from the environment.”
 Because the Earth is spherical, this means that some parts of the Earth receive more solar energy per unit of area than others. This explains why tropical regions located close to the equator are more abundant in both plant and animal life than regions that are located further from the equator – provided, of course, that the organisms have access to water. It also explains why, even in more temperate climates, there is much more vegetal and animal growth during summer, when the sun’s rays shine more directly, rather than obliquely, on the land, and hence each area of land receives more solar energy than it does in winter. Of course, humans have interfered with this natural system by, among other things, transporting large amounts of stored energy from warmer regions to colder regions, and by burning ancient forms of converted solar energy, in order to maintain a greater quantity of life, primarily human but also certain forms of plant and animal life, such as in greenhouses, in less hospitable regions and seasons.