Humans are alone in the natural world (as far as we know) in the richness of our dealings with other members of our own species and in the fact that we have a complex language with which to negotiate these interactions. We have evolved a sophisticated suite of concepts and intuitions, and a correspondingly complex brain to help us succeed in this challenging social environment.
But as the products of evolution (whether biological or cultural), notions such as morality, justice, love, beauty, knowledge, truth, duty, loyalty and so on are only required to be useful heuristics. They are usually not, in their “native” states at least, the result of a robust foundation in rational analysis. These concepts are instead understood and recognized on a basis best described by Justice Potter Stewart with the famous words “I know it when I see it” (regarding obscenity). 
Unfortunately, this is hardly a satisfactory situation for either law enforcement or eroticists. If obscenity is to be censored, then it would be helpful to have a more precise account of what is obscene. Presumably Justice Stewart will not always be available for consultation, not least because he is deceased. In this particular case, it may be sufficient to legislate specifically for certain kinds of depictions, but even so one runs into occasional legal nonsense, as with the decision in Australia that drawings of the Simpsons having sex could be considered illegal child pornography. 
Ideally, this is where philosophy comes in.
In my view, one of the major tasks of philosophy, not only in the interpretation of law but in the resolution of all kinds of conundrums, is to put our intuitions on a firmer footing, and so a substantial body of philosophical work is devoted to debating how we ought to define and delineate these concepts. For an example, we need look no further than our own Massimo Pigliucci’s latest book (co-edited with Maarten Boudry), Philosophy of Pseudoscience: Reconsidering The Demarcation Problem, which seeks to pin down the demarcation between science and pseudoscience and in so doing proposes a robust account of each. 
Some of our natural concepts have been contentious for millennia. For instance, ever since Socrates , and perhaps before, the foundations of morality have been a topic for philosophical debate, and very little has been settled. Attempts to define what is moral have fallen into four major camps: those who define morality in terms of virtue, those who talk in terms of maximizing well-being or utility, those who believe that morality consists of following commandments, and those who deny that moral questions are meaningful at all.
Other concepts seem at first to be easier to define. Plato’s take on knowledge as “justified true belief” stood for over two thousand years. Even so, some issues with this were brought to light by Edmund Gettier as recently as 1963, in a paper which illustrated with examples that this definition fails in certain unusual scenarios.  One way for a definition to fail might be its incoherence, but this is not what Gettier showed. Instead, he demonstrated that there are cases of justified true belief which we would nevertheless intuitively hesitate to call “knowledge.”
And so, in general, it seems that the definitions proposed by philosophers for natural human intuitions can fail if they do not accurately reflect those intuitions.
For Gettier and perhaps for most philosophers, introspection is considered a sufficient means of assessing human intuition. It was after all Gettier’s own intuitions regarding knowledge that he found to disagree with Plato’s definition, while the intuitions of his colleagues were consulted during peer review. However, philosophers do not have a monopoly on human intuition, and I see no reason to privilege the intuitions of philosophers. To provide a solid foundation for the philosophy of natural human concepts, we should have a more robust way to discover and document human intuitions across and within different cultural milieus.
And this is where experimental philosophy comes in.
Experimental Philosophy  (sometimes abbreviated as XPhi) is an emerging discipline which seeks to study human intuitions by collecting empirical data by conducting surveys or psychological experiments. It might, without too much distortion, be characterized as introspection with a sample size greater than one. It can therefore provide a relatively objective, empirical and quantifiable basis for the premises and assumptions philosophical arguments are built on, at least as compared to the traditional alternative which consists of assertions derived from the introspections of individual philosophers. This article will not delve too deeply into what XPhi is or how it works. All that we need to know is that one of its major areas of concern is what laypeople believe regarding the intuitions which philosophers analyze, and that some philosophers regard these beliefs as irrelevant to their work.
Last year, Massimo Pigliucci revealed himself to be such a philosopher.  In his critique of Experimental Philosophy, he noted that while XPhi might tell us what laypeople think about “knowledge,” what philosophers should really be interested in is what other philosophers believe and argue. In the viewpoint exemplified by Pigliucci, the intuitions of laypeople are simply irrelevant to working philosophers, just as mathematicians do not care what laypeople think of Fermat’s Last Theorem. Philosophy is seen as a technical field like mathematics, studying topics about which ordinary people are not expected to have any special insight.
However, in my view this misses the point that, unlike mathematicians, what philosophers are analyzing are those same lay intuitions. There is no syllogistic argument to justify Plato’s definition of knowledge, nor can there be for definitions in general. Definitions are simply declared by fiat, and if a coherent definition is rejected it can only be because it contradicts our intuitive understanding of the defined term. Gettier rejected “justified true belief” because he found that it didn’t match his intuitive concept of knowledge, an intuition he came by not through his philosophical training, but simply by virtue of his membership of the human race (and more specifically of his culture). If Gettier had relied not only on introspection but also conducted a survey and found that few other people (whether philosophically trained or not) viewed the Gettier cases as problematic, he would have had no grounds to find fault with Plato. The fact that his paper has gained traction is only because his own intuitions happen to have coincided with those of most people.
There is certainly a place for technical definitions of technical concepts, but these should be given appropriate technical names and not confused with the natural human intuitions they seek to model. To do otherwise leads to confusion, grandiose claims and failure to communicate effectively. For example, Sam Harris is often criticized for claiming that science can determine human values, but had he instead made the more modest claim that science can guide consequentialist morality (as he actually argues in his book), I suspect the philosophical community would have met his thesis not with derision but with bemused indifference.
Richard Carrier’s account of objective morality  is less well known but is an even clearer example of the problem. Carrier seems to believe he has found the definitive answer to the question of objective morality by essentially taking morality to mean rational self-interest. Carrier’s argument, in condensed form, is that morality is what one ought to do, and what any rational agent ought to do is to maximize its own satisfaction. He suggests that the best way to maximize satisfaction is to be kind, generous, considerate, charitable etc, and that when people behave otherwise they are simply mistaken about which choices will bring them happiness.
Perhaps Carrier has a point on this, but it could be that he is wrong in his assumptions. It is plausible that, at least for some people, satisfaction might best be achieved by behaving in ways which are widely regarded as immoral. Carrier is quite doubtful of this possibility, but untroubled even should it prove to be true. In Carrier’s view, which choices maximize satisfaction is simply an empirical question, and if lying, cheating and stealing is the way to go about it, then that is what is moral for that person. If laypeople disagree, they are simply wrong.
In his own words: “Maybe we shouldn’t always be concerned about the welfare of others. If that’s the fact, then you have to live with it. But whether it’s a fact has to be determined. Empirically.” 
What he is saying is that, in the unlikely event that we find that selfishness leads to happiness, then morality dictates that we ought to be selfish. To me, this is clearly nonsense, and yet it is interesting because I think it arises from much the same kind of thinking as seen in the rejections of Experimental Philosophy from those such as Pigliucci: that philosophy is the study of technical concepts with technical definitions and that lay intuitions have nothing to do with it.
Carrier’s mistake is to overlook the simple fact that morality is a human intuition, so any definition of morality which does not agree with that intuition cannot be accepted. The best we can say is that Carrier has defined something, but to call it “morality” is an error. If no coherent universally-accepted definition of morality is possible, then that is the fact we have to live with. We need to accept it and move on.
In my view, the opinions of lay people are not only relevant but at the heart of some debates, not least that surrounding free will. In the recent back and forth between Sam Harris and Daniel Dennett, the two agree on all the facts of the matter, but disagree on whether the term “free will” is appropriate for the physically-determined actions of biological robots. Harris says it is not, because true “free will” is the libertarian kind which is incompatible with naturalism . Dennett says it is, because the “free will” worth wanting is the one that actually exists and that can be used to justify moral responsibility on consequentialist grounds .
It seems to me that the two are really arguing about terminology. On this particular debate, I side with Harris, but only because, by introspection, I find that the concept evoked in my mind by the term “free will” is not compatible with naturalism. Both men make claims about what the wider public understands by the term, and so it seems to me that the debate ought to be settled with a little experimental philosophy. If Dennett is right that his account of free will matches the intuitions of the public, then he is right to call it so. If he is wrong, then he should call it something else.
Before we finish, let us return to the demarcation problem for a moment. Suppose, hypothetically, that the definition of “science” proposed by Pigliucci and Boudry were not accepted by the majority of scientists. It seems to me that they could respond in at least two ways. One tactic might be to dismiss the scientists as non-philosophers, to maintain that the proposed definition of “science” is technical and not to be confused with the term as used outside of technical philosophy. This would achieve little but to consign their book to irrelevancy. I submit that a better approach would be to survey the scientists and to probe the ways in which the proposed definition failed in the hopes that a better definition could be formulated. This is experimental philosophy in action.
In conclusion, philosophers should feel free to use whatever technical terms and definitions they find useful, but if they ever attempt to define terms which are in general use, and especially if they make use of such definitions in the public space, then they need to know that their definitions match the public understanding of these concepts, or if they do not, they need at least to know that they do not.
And for that, they need experimental philosophy.
Mark O’Brien is a software developer and amateur philosopher who despite never having achieved anything in the field has an unjustified confidence in his own opinions and sees it as his sacred duty to share them with the world. The world has yet to notice. You might very well think that his pseudonymous alter ego is a regular on Scientia Salon, but he couldn’t possibly comment. He is Irish and lives in Aberdeen, Scotland.
 Justice Potter Stewart, concurring opinion in Jacobellis v. Ohio, 378 U.S. 184 (1964).
 Judge rules characters in Simpsons-style sex cartoon are child pornography.
 Philosophy of Pseudoscience: Reconsidering The Demarcation Problem, by Massimo Pigliucci and Maarten Boudry, Chicago Press.
 The Stanford Encyclopedia of Philosophy on Socrates’ Euthyphro Dilemma.
 The Stanford Encyclopedia of Philosophy on Knowledge.
 The Stanford Encyclopedia of Philosophy on Experimental Philosophy.
 Rationally Speaking: Philosophy is not an Elephant, by Massimo Pigliucci.
 What exactly is objective moral truth?, by Richard Carrier.
 A comment from Richard Carrier in the discussion of .
 Free Will and “Free Will,” by Sam Harris.
 Reflections on FREE WILL, by Daniel C. Dennett.