The difficult we do right away. The impossible takes a little longer. Engineer's joke.
It's only impossible if you stop to think about it. The Pirates! Band of Misfits (2012 movie).
Unlimited optimism.Judging from my email and occasional eavesdropping on web forums, I conclude that there are many people in this technological world who sincerely believe that "anything is possible" in science and engineering. They have seen science produce wonders previously thought impossible, and are confident that, given enough time, science will conquer all of the challenges we now find unimaginable. They love to quote the line "They laughed at Galileo, didn't they?" (Or, substitute the name of any renowned scientist or inventor.)
They dislike any suggestion that particular things might be impossible, especially particular things they desire very strongly, such as perpetual motion machines and over-unity machines. Sometimes this is expressed as an almost religious conviction that if mankind needs something very badly, then somehow, someday, some clever person will find a way to accomplish it. It is a "faith in the possibility of unlimited progress". So, we are running out of fossil fuels? Not to worry, someone will find a substitute fuel in great abundance. Is our population growth threatening to exceed the resources of earth? No problem. By the time that gets dicey, we will be colonizing the planets, and eating delicious synthetic food made from industrial waste. This sort of thinking has (apparently) always worked for us in the past, so surely, some think, such progress will continue forever.
I submit that this excessive faith in science stems from a profound misunderstanding of science. It is a selective reading of history, concentrating on past instances of success, without examining why those successes were possible. Also, people love stories (often only partly true) of cases where pessimistic predictions were found to be wrong. Very often these pessimistic predictions were casual hunches and not carefully thought out analyses, and some of these stories are simply urban legends. The "bumblebees cannot fly" legend is one such case.  Such negative predictions, especially when made by prominent persons of history, make entertaining reading, and I have a collection of them on my website, titled It'll Never Work. Equally entertaining are past optimistic and fantastic predictions of future developments that haven't come about, and never will. Several published books have mined those as cautionary examples of the dangers of overconfidence in science and technology. 
The chimera of over-unity devices.Throughout the history of technology, people have been fascinated with the possibility of a machine that would do useful work while requiring no energy input, or at least much less energy than conventional machines that burn fossil fuels, or use "natural" sources such as wind and water. Their goal is a machine that puts out more energy in the form of useful work than it takes in, a hypothetical device that they call an "over-unity" machine, because its energy efficiency would be greater than one. Sometimes this is loosely called a "perpetual motion machine" because if some of its output energy were used to provide the input energy, it could run forever and still put out some useful work. Needless to say, no one has achieved this goal.
One might have thought people would give up this effort once scientists formulated and then understood the laws of thermodynamics, which tell us that energy is strictly conserved in any mechanical device, whatever its detailed construction, whether it be strictly mechanical, or electrical or magnetic, or whatever else you might conceive.
How can we know what's impossible?But ever-optimistic inventors saw these laws as a challenge. They had many rationalizations for their optimism. Surely the laws must be wrong. How can scientists be so arrogant as to declare anything to be impossible? We haven't tried everything yet. How can you know that some future clever invention couldn't exploit a loophole in known science? Science has been wrong in the past, maybe the thermodynamics laws are incorrect, at least in some as-yet-unobserved case. In particular, science could be wrong in the case I want to be true.
So, the perpetual motionists forged on, tinkering and fiddling with wheels, gears, magnets and fluids, each hoping to be the first to stumble on the secret of unlimited energy. "It must be possible," they said, "because mankind desperately needs it." Each year patents are issued for devices that any competent physicist would find laughable. The Internet has many websites and forums describing incredible devices, along with claims of the potential of achieving incredible resultswith a little more refinement and tinkering. These accounts are frustratingly short of details and experimental results. Indeed, they sometimes describe a machine as "working" that hasn't yet been built, even in prototype.
Where's the secret hiding?So much of science, particularly classical physics, is so well understood that we know there's no loophole there to allow energy creation from nothing. So some hopeful inventors look to the areas of physics that are less well understood, and especially poorly understood by non-physicists. Some of the perpetual motionists reject the criticism that they are trying to defy the laws of thermodynamics, or any other laws of physics. They say they are hoping to make a machine that will somehow tap a "natural" source of energy (free energy) in the universe that we haven't discovered yet. A successful over-unity machine would in fact constitute the discovery of that energy. The machine itself would be a "detector of invisible free energy".
Others pin their hopes on certain notions currently popular in speculative theoretical physics, such as "dark energy", "dark matter", and "zero point energy". After all, they say, these are "another form of energy" and we know that even matter can be converted to energy. This new energy must be there, free for the taking. These folks propose, and some even build, machines that are superficially indistinguishable from previous attempts to get energy from nothing, only these folks claim that the energy does not come "from nothing" but from energy that's invisible and all around us.
Personally, I think those who seek to make such "free energy" machines are destined to fail. But my reasons aren't easy to explain. Since I have been a physicist my entire career, I have a different perspective on these questions than non-physicists do. I am well aware that science is not yet complete, and that present-day scientific laws and theories will surely be modified and improved in the future. We don't know everything yet, and probably never will, but it doesn't follow that anything we might imagine "could be true". Perpetual motion believers hope that there's a flaw or loophole in our understanding just such as to allow clever inventors to exploit a "sea of energy" that they imagine must pervade the entire universe. The catch is that (1) that "sea" of zero point energy the physicists postulate is locally very dilute, and only tiny amounts of this energy would be within the "grasp" of such a machine, and (2) nature has certain "gotchas" in its laws that provide no way to extract even that energy to produce useful work. In fact, the laws governing it specifically prevent this. Finally, even if we are wrong about those "gotchas", present scientific models give us no clue how to get around them, so inventors are like blind men fumbling around in the dark, hoping that something might magically work if they tinker with it long enough. Along the way they are encouraged when they see some result that was unexpected (to them), and seemingly not in accordance with their (limited) knowledge of physics. They proclaim such observations as "breakthroughs".
In their enthusiasm, the inventors seem to think that "environmental" energy comes completely free. Every such energy source we know of requires expenditure of energy to extract it from nature. Extraction methods aren't perfectly efficient, and no one knows how to make a perfectly efficient system of any kind. If they did, that system itself would be a perpetual motion machine, even if it didn't have over-unity performance. It would turn forever without producing any useful work.
Another thing that perpetual motionists overlook is that the physicists' "zero point energy" is a "phantom" energy, a mathematical fiction that is part of the mathematical theory of quantum mechanics, and is not necessarily equivalent or convertible to what we call "real" energy (of a kind that can be converted to useful work). These "energies" are something of the character of the older concept of "electric field lines" that are shown in textbook pictures, depicting how charges act upon other charges, or the "gravitational field lines" from masses. Physicists know that field lines are not "real" entities in space, but only a convenient mathematical concept to help us visualize what's going on and predict what would happen if a charge or mass were placed at a certain place. No one would think that we could reach into "empty" space, grab a handful of field lines, and put them in a box.
Even people who do not for a moment accept the possibility of perpetual motion sometimes fall into this philosophical trap. They say, "Perpetual motion can't work because you can't create energy from nothing." That is an empty argument. They are thinking of energy as "stuff" or a "substance", then implicitly assuming that stuff is indestructible and cannot be manufactured from something else." But energy isn't material stuff like matter. Energy is a convenient mathematical concept for doing the "bookkeeping" as we observe how things interact and influence each other. Energy is an "accounting" scheme for describing the behavior of physical things.
Mesmerizing magnets.Devices using magnets are particularly popular these days. I get many designs for perpetual motion machines using magnets. These folks are having fun tinkering with magnets, but haven't produced any results that confirm their hope (conviction) that such devices are capable of producing more energy output than input.
Why does the behavior of magnets fascinate so many people? Probably because magnets are not part of our daily experience, and in certain situations behave in ways that are surprising (and seem "magical") when first observed. Those who do laboratory physics and electrical engineering have become familiar with the behavior of magnets, and know the laws of their behavior. Making predictions with those laws can be complex. When magnets interact and move relative to each other, the shape of the magnetic field around each one changes. Still, the bottom line is this: we know very well the laws they obey, and no experiment we've ever done with magnets has shown the laws to be violated. Furthermore, the laws can be shown, mathematically, to be fully consistent with strict conservation of energy and momentumwith no exceptions. A vast amount of experimental evidence confirms that. If there were flaws in these laws, that fact would surely have shown up in the functioning of one or more of the electromagnetic machines (motors, generators, etc.) that keep our industrial technology humming along. So the bottom line is that those who hope for a magnetic perpetual motion machine are doing so because they do not yet understand the physics and behavior of magnets.
How can we be certain nature is always lawful?
But there are philosophical questions. These have to do with the concept of nature's lawfulness. Does nature operate according to strict and regular laws (even if we do not yet know all of those laws)? If nature is perfectly lawful, we can predict its behavior in particular situations, although not perfectly (since our measurements are never perfect). Or does nature sometimes do something "capricious" and unpredictable? In that case, our predictions might be far off the mark, and the event would qualify in some people's minds as a "miracle".
In physics, when something like an unexpected scientific observation turns up, more careful examination eventually shows that our predictions were mistaken. Either we blundered in the calculation, or we overlooked a subtle condition or variable. But in a few much-publicized cases, some previously accepted law was at fault, and the observation tells us something new that may lead to refinement of a that law. Then we realize that nature was behaving lawfully after all, we just hadn't gotten the law completely right. No such case, in the whole history of science, has convinced us that nature has ever, not even once, behaved in a capricious manner. So we conclude, at least as a working hypothesis, that "nature is strictly lawful" and it is our job to keep trying to express those laws better and better. Actually we are trying to construct mathematical laws that fully describe nature in all cases. But we aren't so arrogant as to assume that these laws we invent are "nature's laws". We know, from past examples, that two different sets of laws may describe nature equally well, and to say that one set is "better" or "truer" than the other is saying too much. So the laws we formulate to describe nature may change, but nature's lawful behavior continues unchanged.
Lest my meaning be misinterpreted, I am talking about basic laws here: laws like Newton's laws of mechanics, Maxwell's laws of electrodynamics, the laws of thermodynamics, etc. Sometimes, things unexpected to us occur in complex situations, without violating any basic laws. It may rain when no rain was predicted. These are situations where outcomes depend on the interaction of so many basic laws of nature in combination that we can't possibly know the relevant variables well enough to make precise and fully reliable predictions, or we don't have the computing power to process the data.
Even in complex situations as this there are some physics laws that can be tested, after the fact, and shown to have been fully obeyed. No matter what the weather does tomorrow or next week or next year, it will never violate the laws of thermodynamics, the conservation of energy, the conservation of momentum, the conservation of angular momentum, etc. There is a class of physical laws, called "conservation laws" that allow us to make definite, measurable predictions about entire complex systems without knowing everything that's going on in the system. Physicists like such laws, because of their robust character. They allow us to make correct predictions even though we have some ignorance of the system. And, so far, nature has not let us down.
These conservation laws are not laid down by scientists as dogma by fiat. Nature has imposed them upon us, and we express them in words and mathematics because they are so useful. Such laws are inferred (generalized) from the whole history of our observation of what nature does, and from experiments done in laboratories around the world (and by observations made by astronomers in the larger laboratory of the universe). They have been tested not only experimentally, but also by forging the mathematical links between them and the other fundamental and well tested laws in all fields of science, to convince ourselves that they constitute a logically consistent and comprehensive framework for describing nature.
That's why we confidently say, "No one will ever make a machine that outputs more useful work than the amount of energy input to it." Such a machine would violate conservation of energy, and we can show mathematically that it would also seriously violate Newton's law F = ma, Newton's third law, and therefore also violate conservation of momentum. These violations would not be small and easily overlooked. They would be large and serious violations, easily observed. Yet no such violations of these laws have ever been observed in the history of science.
Universality of physical laws.
We must mention another important point, one that is seldom discussed in modern textbooks. That is the universality of physical laws. In the early history of science laws were assumed to have applicability only to limited areas of experience. A law about water was not necessarily assumed to also apply to some other liquid. Laws that were true on earth did not apply to the sun, moon, and stars. In fact, Aristotelian physics declared that the laws of the terrestrial realm (on earth) did not apply in the celestial realm (the heavens). This view was held till the time of Newton. Newton is credited with unifying the laws of motion of the moon and planets with the mechanical laws of motion on earth. His famous diagram relating the motion of a hypothetical cannonball with the orbit of the moon nicely illustrates this. Newton's law of gravitational attraction accomplished this unification, and is called "Newton's Universal Law of Gravitation", because it is assumed to apply everywhere in the universe and at every time in history or in the future. That is, the very same equations apply on earth as in the heavens, and in the past and future. Now when put that way, that's a rather audacious claim, for we haven't yet tested the equations everywhere and at every time. Of course we are testing and confirming them every day, as scientists observe and measure motions of everything they observe in the heavens, and as NASA computes orbits of spacecraft and space probes and puts the results to the test with every space vehicle that is sent from earth.
Science continually tests the universality of laws, as well as the logical unity of laws. So far the laws have met the tests with flying colors. Of course unexpected facts of nature continually turn up, and some of these were not anticipated by the old laws as they were previously formulated. For example, research on atomic physics taught us that the law of conservation of energy needed to be amended to the conservation of mass-energy. And later, relativity showed that momentum and energy are part of an energy-momentum four dimensional vectorwhich is a conserved quantity in a closed system. So our physics at any given time may not yet have the universal laws of nature formulated perfectly, but as we discover new facts and devise new laws and modify the old laws and theories, it still appears that nature itself does operate with universal lawfulness and regularity. We just haven't yet learned all the details.
Every advance in physics has confirmed that "Nature abhors macroscopic perpetual motion," and that "no device can put out more useful macroscopic work than its energy input".
None of this gives any encouragement to perpetual motion enthusiasts. Some may still hope that there's some place or special situation in the universe where over-unity machines are possible. Perhaps over-unity machines only work on Tuesdays at 3:15 PM at a particular spot on Mars. Of course that's an absurd suggestion. But, seriously, if a particular one of our physical laws is violated at some particular place and time, how do we interface a machine operating there with the rest of the universe where the ordinary laws are scrupulously obeyed? You can hypothesize all sorts of fantastic ways to get around this, but there's no hard evidence supporting any of them, and not a clue how to implement them. 
Still, the perpetual optimist asks, "How can scientists be so sure of the validity of laws that they claim apply to things that haven't yet been seen?" It's a reasonable question. Of course part of the answer is that we can't be absolutely certain of any particular law's applicability to any particular, as yet unobserved, device or process. No law is claimed to be perfect. But some are so thoroughly tested and precisely confirmed in so many diverse situations that it would be foolhardy to bet against their being universally valid. Also, we must distinguish between well-established and well-tested laws, and other, less well-validated ones. We must also note that some laws are so thoroughly integrated into the logical/mathematical scheme of other laws that the likelihood of all of them being seriously in error is virtually nil. Underlying all of this is, of course, the confidence that nature, though sometimes subtle, will not play games with us by changing the rules just when we think we've figured them out.
The unified web
To return to the perpetual motion enthusiasts... Even though they seldom articulate them, they have some philosophical preconceptions in common. While they recognize that nature behaves predictably and lawfully, they hope that there's some deficiency in our understanding of these laws that would allow for unlimited production of useful work greater than the energy used. They fail to appreciate the implications of this, if it were so. They think that it could be done without modifying any known and reliable laws of physics, such as Newton's laws. They do not appreciate the mathematical/logical interdependence of all of these laws. They treat each law of physics as an independent and separate entity. If there were a machine that, say, put out 10% more useful work than its energy input, that could only be so if all of the fundamental laws of textbook physics were seriously wrong. Do they really think that is so? Perhaps not, but they still seem to think all the laws are independent of each other.