Voting has opened for Prospect's 2015 World Thinkers poll (you can cast your votes here). With the help of several of our regular contributors and other friends of the magazine, we've assembled a list of 50 thinkers who we believe are "engaging in original and profound ways with the central questions of the world today". You'll notice that we've given credit for "their influence over the past 12 months"—which means that people didn't qualify by eminence alone—and for their continuing significance for "this year's biggest questions" (in economics, science, philosophy, cultural and social criticism and in politics).

The poll, which began in 2013 (when it was won by Richard Dawkins, who fell foul of our strictures on currency the following year and disappeared from the list), is the descendant of a list of 100 "global public intellectuals" that Prospect first ran in 2005, in collaboration with the American journal Foreign Policy (the exercise was repeated in 2008). There are some fascinating and, I think, instructive differences between the way the poll was run then and now.

The first and most obvious difference is one of nomenclature: the 2005 and 2008 lists were lists of "global public intellectuals"; today, we're soliciting votes for "world thinkers". Those who drew up the 2005 list were strikingly frank about the difficulty of settling the hoary question of what a "public intellectual" is, exactly. "The irony of this list," they wrote, "is that it does not bear thinking about too closely." That, you might say, was part of the problem, since the criteria they did manage to spell out were ambiguous to say the least: "What is a public intellectual?Someone who has shown distinction in their own field along with the ability to communicate ideas and influence debate outside it."

As I wrote in 2006, in a review of Stefan Collini's excellent book about intellectuals in Britain, Absent Minds, in which I commented on the Prospect/Foreign Policy list, there appeared to be two distinct models of the public intellectual in play here. "In one," I suggested,"call it the 'popularising' model, the intellectual makes his [or her] work accessible to an audience of non-specialists." Steven Pinker and EO Wilson, who were both on the 2005 list, are "public intellectuals" in this sense. In the other model, a "political" one, "the intellectual takes advantage of his [or her] academic prestige in order to pronounce on matters of broad public interest" (Noam Chomsky, who won the poll, was a public intellectual in this sense). (It's worth noting, incidentally, that Pinker, Wilson and Chomsky didn't make it on to this year's list.)

For one thing, the 2005 criteria were ambiguous between those two models. For another, they appeared to rule out thinkers "engaging in original and profound ways with the central questions of the world today" who weren't necessarily communicating their ideas to an unspecified "public". (On the slipperiness of the idea of the "public", see this excellent new essay on "What's Wrong With Public Intellectuals" by the writer and critic Mark Greif. Greif is particularly interesting on the way that, as I put it in in back in 2006, "the habit of adding the world 'public' to the word 'intellectual'… is almost invariably expressive of anxieties about the academicisation of intellectual life and the perceived inability of specialist intellectuals to make contact with a non-specialist public." In assembling this year's list, you'll see that we've been fairly relaxed about the "specialisation" problem.)

None of this is to say that the criteria we've applied this time round are perfectly conceived—far from it. This isn't an exact science, after all. And our understanding of what today's "central questions" are reflects our own preoccupations more surely than they do any objective standard.