Along with writing a leading article on faith schools and travelling to Texas to conduct what turned out to be the final interview with fellow atheist Christopher Hitchens, the evolutionary biologist Richard Dawkins has penned an exclusive essay for his special guest-edited issue of the New Statesman.

In "The tyranny of the discontinuous mind", Dawkins wonders why we cling to absolutes of yes and no, black and white, rich and poor; pretending not to see the millions of grey areas in life. These absolutes, he argues, distort reality:

What percentage of the British population lives below the poverty line? When I call that a silly question, a question that doesn't deserve an answer, I'm not being callous or unfeeling about poverty . . . My objection is to the very idea of a line: a gratuitously manufactured discontinuity in a continuous reality.

. . . For legal purposes, say, in deciding who can vote in elections, we need to draw a line between adult and non-adult. We may dispute the rival merits of 18 versus 21 or 16, but everybody accepts that there has to be a line, and the line must be a birthday. Few would deny that some 15-year-olds are better qualified to vote than some 40-year-olds. But we recoil from the voting equivalent of a driving test, so we accept the age line as a necessary evil. Yet perhaps there are other examples where we should be less willing to do so. Are there cases where the tyranny of the discontinuous mind leads to actual harm, cases where we should actively rebel against it? Yes.

Dawkins goes on to consider a variety of these absolutes -- where a blindness to intermediates may constrict or condemn us -- beginning with the arguments proposed by anti-abortionists:

There are those who cannot distinguish a 16-cell embryo from a baby. They call abortion murder and feel righteously justified in committing real murder against a doctor - a thinking, feeling, sentient adult, with a loving family to mourn him . . .

It is amusing to tease such absolutists by confronting them with a pair of identical twins (they split after fertilisation, of course) and asking which twin got the soul, which twin is the non-person, the zombie. A puerile taunt? Maybe. But it hits home because the belief that it destroys is puerile, and ignorant.

His own field of evolutionary science provides Dawkins with a conundrum for those intent on seeing the world in binary terms:

If a time machine could serve up to you your 200 million greats grandfather, you would eat him with sauce tartare and a slice of lemon. He was a fish. Yet you are connected to him by an unbroken line of intermediate ancestors, every one of whom belonged to the same species as its parents and its children . . .

If your theology tells you that human beings should receive special respect and moral privilege as the only species that possesses a soul, you have to face up to the awkward question of when, in human evolution, the first ensouled baby was born.

Dawkins also considers the issue of race, and our tendency to use distorting cultural labels as if they were dominant genes. Why, he asks, when Colin Powell and Barack Obama have both black and white ancestors, do we describe them as black -- rather than white?

Our language is ill-equipped to deal with a continuum of intermediates. Just as people must lie above or below the poverty "line", so we classify people as "black" even if they are intermediate. When an official form invites us to tick a "race" or "ethnicity" box, I recommend crossing it out and writing "human".

Before returning to the problem that such absolutes pose for scientists -- as well as journalists and lawyers -- Dawkins argues that dividing lines allow for distortions and misrepresentations in our politics:

This "winner takes all" system was shown up in all its fatuousness in the 2000 election when there was a dead heat [between US presidential candidates George Bush and Al Gore] in Florida. . . . [the] electoral college [system,] in which each state has an indivisible block of members, either all Democrats or Republicans, no matter how close the vote, is a shockingly undemocratic manifestation of the tyranny of the discontinuous mind.

This essay in full can be found on page 54 of the Christmas double issue of the New Statesman.

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