Parmenides' fallacy is a term invented by author Philip Bobbit to describe a mistake people often make when they are evaluating particular courses of action, especially in politics and international relations. Parmenides' fallacy "occurs when one tries to assess a future state of affairs by measuring it against the present, as opposed to comparing it to other possible futures." This means that when one is contemplating a particular course of action - say, whether to launch a war - the important comparison to make isn't between the present situation and what the situation will be like five years after the war. Rather, the important comparison is between the different situations that would obtain in five years if one did launch the war and if one didn't.
This may seem counter-intuitive; it certainly seems difficult. After all, predicting the future with any degree of accuracy is one thing our social sciences are wholly incapable of. The standard trick is simply to extend current trends into the future on the assumption that they will continue, which may be appropriate in some cases but is woefully inadequate in many. Nevertheless, an attempt at this sort of analysis must be made before any major political or foreign policy decision. One cannot assume that time will stand still if one does not act; in fact, faced with an ever-changing world, not acting is just as significant an action as acting.
A number of examples will suffice to illustrate the point. Take firstly American assistance to the Afghan mujahadeen to resist the Soviet occupation of their country. We will see that Parmenides' fallacy has some relation to the concept of "blowback", which refers to the unintended consequences of covert intervention in a foreign country. It is often charged by critics of the CIA that by providing weaponry to the mujahadeen, they gave rise to the Taliban, and hence created numerous problems for the Western world. This is taken to be a sufficient indictment of them providing the aid in the first place. Similarly, an international relations student at Cambridge once tried to convince me of the irrationality of Israeli meddling in Lebanon because in the 2006 Lebanon War the Hezbollah were able to turn on Israel some of the weaponry that had been given to another militia in Lebanon by Israel in the past.
We can see quite easily that, if this is the sole basis of the critics' argument, they have succumbed to Parmenides' fallacy. Sticking to the example of the mujahadeen, we can see that they conceive of Washington's choice as either simply having been to help the mujahadeen and in the future face the Taliban threat, or to not help them and hence not face the Taliban threat. They have not considered what the present day might be like if Washington had not helped create the Taliban, but had instead allowed the Soviet Union to occupy Afghanistan indefinitely.
Arguably, in this case there would have been no Soviet collapse, and the Red Army might have pushed further into the Middle East and elsewhere. Absent the sapping impact of a long and unwinnable war in Afghanistan, the Soviet Union could have continued to exist and the superpower competition could eventually have brought nuclear apocalypse to the world. Even if one quibbles with these conclusions, it is on these terms that the debate needs to be had, not by simply observing that now there is a Taliban and in 1979 there wasn't, and hence the policy was a failure. History would have marched on from 1979 regardless, and it is these other possible futures that we need to consider in evaluating a policy decision. That a decision caused pain is not enough to condemn it if it can be demonstrated to have also prevented an equal or greater amount of misfortune in other ways.
To take a more recent and hence controversial example, look at the war in Iraq. The war in Iraq was explicitly based on a rejection of Parmenides' fallacy, as we can tell when we recall it was called a "pre-emptive war". The war was intended to pre-empt possible futures in which Saddam Hussein could, depending on who you asked, once again commit large-scale human rights violations, provide weapons of mass destruction to a terrorist group, or seek once again to establish hegemony over the Middle East. When one considers whether the war was a success in these terms, one ends up having quite a different debate to simply observing that the war has been a disaster which had not occurred in 2003, and hence it would have been better not to do it. The trick isn't to compare our situation now to our situation in 2003, but the situation we would be in now if we hadn't launched the war compared to one we are in.
Briefly, a justification of the war on these grounds would cite two main factors. The first was the unravelling of the sanctions regime on Iraq in 2003, which Geoge W. Bush had come into office promising to overhaul as it was obviously breaking down and Saddam was engaging in widespread cheating, including smuggling and fraud in the oil-for-food programme. The sanctions, when manipulated by Saddam, were killing 3,500 Iraqi children a month, according to UNICEF. This was obviously undesirable in itself - and is no longer happening after the war - and was also putting strain on support for the sanctions regime among all the countries required to implement it. The second factor is Saddam's now well-documented intent to restart his WMD programmes once the sanctions were lifted, which seemed certain to be on the cards before the march to war.
I don't offer this argument to try and convince you of the war was the right thing to do - that remains an open question - but to again re-iterate the main point. The choice wasn't between the situation we have now and the weak, hemmed-in Saddam Hussein of March 2003, but the situation we have now and a much-stronger, well-armed, expansionist Saddam Hussein who would now currently be ensconced in Baghdad. (We would also be faced, incidentally, with a weaker Iran). There may have been better policy options for preventing the latter outcome than war, but it is in this realm that the argument needs to be had, not by simply assuming history would have stood still in March 2003 had we not invaded Iraq.
And it is by applying similar logic that all mature policy debates need to be conducted. It makes debate much more difficult, and puts a heavy burden on our ability to analyze possible futures. Above all it encourages a deep humility that we ought to have in the face of the awesome complexity of human affairs and their total resistance to planning, especially when we are deciding whether or not to unleash violence.