On Evil, By Terry EagletonThe Uses of Pessimism, By Roger Scruton

For well over three decades, Roger Scruton and Terry Eagleton have as public intellectuals put on an entertaining non-stop show as the lion and the unicorn of competing ideologies. In the blue corner, with a training in philosophy sharpened by his expertise in music and architecture as well as bare-knuckle bouts of political controversy, Scruton has punched hard for traditional conservatism and the rooted laws and customs that sustain it. In the red, his grounding in Marxist criticism and theory handily reinforced by theology, ethics and Catholic social doctrine, Eagleton has proved a champion of the light-footed left that knows how to dance as well as stamp. Now both have published books that, for all their keen differences, together pose a disturbing question. Have lion and unicorn morphed into Tweedledum and Tweedledee? And, if so, has one given up the fight, or have both collapsed in the middle of the ring?

One of these books, for example, aims to develop the kind of argument about human beings and their nature that "remains faithful to many a traditional theological insight". The other dismisses theology as an "entirely phoney" domain. One argues with Aristotle and Aquinas, against modern individualism, that "virtue is by far the fullest, most enjoyable way to live"; it also scorns "leftists for whom pessimism is a... thought crime". The other proposes a morality based on self-critical "habits of forgiveness and irony"; it celebrates civil society not as the gathering of homogeneous people but as "a free community of strangers", and is not bothered by forms of faith since "religion may decline and fragment without damage to the rule of law".

One writer defines original sin, fatalistically, as the unavoidable "fact of being born" into a body in thrall to drives and desires. The other sees it, hopefully, as the creative rebellion of Cain when he slew his brother Abel, "turned his back on the tribe" and set off to found cooperative settlements of citizens. One insists that "the past is what we are made of" and that "throngs of ghostly ancestors lurk within our most casual gestures, preprogramming our desires". The other praises the "collective rationality" of "irretrievably diverse" societies with "the capacity to live in peace and adapt through consent and consensus".

One scoffs at the "mindless progressivism" of that "staggeringly complacent" atheist, Richard Dawkins. The other soberly accepts that history shows "we can... move on from our original nature". And the first voice, in each case, belongs to the radical Eagleton; the second, to the conservative Scruton.

Eagleton's book aims to rescue the reality of evil as a very rare but extreme case of "wickedness" against those liberals or progressives who would discount the idea as a survival from a superstitious age. It accepts that the proud, clever and almost abstract destructive cruelty occasionally practised by people or by states – from Iago to the Nazis – might still deserve the name. It sees evil as a grotesque form of will, even of art: a sort of absolute selfishness built on the denial of our "creaturely" natures, and of our dependence from the womb "on others of our kind": "Pure autonomy is a dream of evil".

Scruton's study seeks to defend "the image of human imperfection" against those utopian and optimistic creeds that dream of a future without conflict or compromise. It suggests that all persons and societies must "recognise limits and constraints... boundaries we cannot transgress", and cherish the slow-cooked institutions that allow us to grow up safe and live in peace. Via "small doses of pessimism", Scruton aims to inoculate us against the fatal fever of utopian ideals. He warns against the "fallacies" of grand top-down designs – from modernist architecture to liberal education, the Soviet Union to the (almost equally deplorable) European Union – that will always end in tears.

From its taste for "discipline and sacrifice" to its admiration for Enoch Powell and detestation of Le Corbusier and masterplanners in every field, Scruton's conservatism is so- familiar – comforting, even – that allies and enemies alike will greet it like an old comrade or sparring-partner. In a nutshell, "The solution does not exist as a plan, a scheme or a utopia. It is the residue of myriad agreements and negotiations, preserved in custom and law".

Put these books together, however, and you see an extraordinary optical effect. Scruton's utopianism of "false hopes" more or less converges with Eagleton's idea of "evil". To Scruton, utopia is also artful, abstract and wilful, a monstrous assault by the inflated ego on reality (both invoke Schopenhauer). It is Eagleton whose concept of history rules out "the possibility of utopia", since "There are certain negative features of the human species which cannot be greatly altered." Scruton concurs that "our virtues and loves are the virtues and loves of dying creatures". A chief trait of evil, adds Eagleton, is its "refusal to accept our mortality as natural... beings".

Yes, when it comes to upfront politics, the pair do disagree in predictable ways. So Eagleton views Islamist terrorism as a wicked but rational riposte to historical injustice; Scruton as an inner-directed quest for annihilation founded on rage and resentment at modernity. Yet it's Scruton who expresses sympathetic curiosity about this mindset, noting that the 9/ll hijacker Mohamed Atta wrote a dissertation on the ruin of the fine Muslim city of Aleppo by "junkyard modernism".

And in Eagleton's Freudian account of the "death drive", Scruton would find an eloquent explanation of self-destructive urges that flame out to consume the world – except that Eagleton dissects not Islamism, but alcoholism. Scruton's utopian, like Eagleton's evil-doer, is drunk on a hatred of embodied reality.

Both thinkers write with consistent clarity and frequent wit. But both harbour neurotic blind-spots. Scruton refuses to accept that we have just witnessed a "crisis of capitalism". He is unable to see global capitalism and Western empire as destructively utopian ideologies of coercion. It is not trivial to ask where he would stand on the proto-totalitarian crimes of uniformity committed by the Spanish Inquisition and its heirs. For his part, Eagleton vainly seeks to exempt the massacres of Stalin and Mao (although "beyond the moral pale") from the hellish category of Nazi genocide. He falls into just the utopian trap Scruton exposes, positing that true history has not begun since – prior to an endlessly-deferred real revolution – humanity remains mired in "rapine, greed and exploitation".

So Tweededum and Tweedledee keep their distance at last. The day of coalition thinking has yet to dawn. Still, it fascinates to see these old combatants ready to share so much terrain: the ground, above all, of the mortal body and mind, forever denied perfection. "Those who sentimentally indulge humanity" – as Eagleton writes, and Scruton might - "do it no favours." For a double dosage of reality, read both books.