No need to keep readers in suspense – the fallacy is to aim for certainty beyond the shadow of doubt! It is very costly because by holding on to the belief that if one lacks such certainty, it's OK to believe this or that and to do this or that, one is wasting enormous resources. And this is the basis of much public policy – especially, since the funds to engage in such fruitless pursuits can be obtained via the extortionist methods of taxation which creates the illusion of no limits. It is no accident that President Obama, for example, has linked his own public philosophy to the idea of hope – as seen in the title and theme of his famous book, The Audacity of Hope (Canongate, 2007). Pursuing what one can only hope for, mostly against all reason, is just how one produces enormous debts, especially when one doesn't need to worry about who will have to foot the expense of such pursuits.
In the sciences, too, this is a major fallacy. For centuries, for example, there has been a debate about whether one can know if other people are conscious. It goes something like this: "No one can enter another person's mind and all one can do is observe behavior, so isn't it possible that everyone who superficially seems to be conscious like oneself is, in fact, mindless? Isn't this possible? Can it be ruled out? Is it certainly so beyond a shadow of doubt? If not, well go for it!"
Well, if the standard of what can be ruled out is that it must be certain beyond a shadow of doubt that it could be, then most of what one imagines cannot be ruled out. Are we certain like that of anything at all? Isn't it conceivable, imaginable, that I am dreaming that I am sitting at my computer now typing away? Can I be sure beyond a shadow of doubt that I am not? Sure, but what of it? Such doubtfulness is utterly pointless, irrational. It is why in a court of law the goal is certainty beyond a reasonable doubt, not a shadow of doubt.
If it were possible to gain certainty beyond a shadow of doubt, it would be impossible spell it out. For doubts can always be imagined past the current ones. What should be the standard is certainty of the kind that withstands doubts that are well grounded, for which reasons exists. So that if I were sitting at my computer and my vision and thinking became fuzzy and around me all were spinning in a fog, then I would have reason to doubt that my belief that I am indeed sitting there would rest on something worth exploring. As it stands, simply fantasizing that I might be out of my mind is a source of paranoia, not sensible concern. And costly visits to a psychologist!
So can we be sure that others are conscious even if we cannot get into their minds and check this out? Yes, indeed, we can – even those who toy with the notion that people might be deny this notion in how they act and live, for example, by writing about the issue for readers they surely know are conscious enough to grasp what they are saying.
The fallacy of wishing for the kind of certainty that is beyond a shadow of doubt shows up everywhere – despite the lack of evidence for extraterrestrial life people and institutions invest enormous resources on searching for it. Despite no evidence for thinking that government stimulus packages can dig a country out of recessions or depressions, politicians and policy wonks keep up the hope that it is possible to do it – getting something out of nothing, to put the idea in its most basic form and thus indicating just how contrary to reality it actually is.
There are two very good books about this issue that should be read far more widely than they are. Shirley Robin Letwin's The Pursuit of Certainty (Cambridge University Press, 1965), and Ludwig Wittgenstein On Certainty (Harper, 1969). The bottom line is that although it is sometimes, rarely, useful to base actions and policies on mere hope – if there is at least some credible reason lined up behind such hope – in the bulk of cases resting public policy and personal aspirations on the fact that one doesn't know beyond a shadow of doubt that a course of action is futile is a very bad idea. And it is very very costly, unless you can steal the funds from others to support such fantastic explorations.