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Readers of New Humanist are all too familiar with the current demands by public intellectuals for a return to Enlightenment ideals, for a defence of secularism, tolerance, atheism and science against a growing number of threats. This has meant scouting out the enemies of reason. Whether these enemies are found within the “cartoon controversy”, in the proselytising of the Christian Right or in the many forms of religious violence and rebellion around the world, they are often likely to be labelled “fanatics”. Yet this epithet, which seems to be so unequivocal, has a long and complex history. Partisans of rationality may want to think twice before bandying about a term which in the past has also been used to condemn Enlightenment values and the struggle for freedom and equality.

It was Voltaire, scourge of unreason and superstition in texts like Fanaticism, or Mahomet the Prophet, and the Philosophical Dictionary, who painted the most enduring portrait of the fanatic as the sworn adversary of the Enlightenment, and conversely of the philosopher as the paladin of tolerance. Fanaticism, for Voltaire, is a political perversion of the religious spirit, a destructive and contagious group fixation on divine prescriptions which will not rest until any contrasting opinion or belief is laid to waste. Though Voltaire excels in enumerating the many faces of irrationality, he finds an underlying uniformity. All fanatics, he writes, “have the same bandage over their eyes.”

There is something comforting about the idea that unreason is such a unified phenomenon, in the belief that there is a clear line between tolerance and fanaticism, and in the assumption that the long list of reason’s enemies, irrespective of geographical space (Hezbollah, Hindutva and the American Christian Right), history (from the Prophet Muhammad to David Koresh) or political orientation (Hitler and Lenin, medieval millenarians and neo-conservative demagogues), all somehow share the same kind of fanaticism.

But what we lazily refer to as the Enlightenment harboured other, more disconcerting interpretations of fanaticism. For Immanuel Kant, rather than representing an external threat to rationality, fanaticism appeared as one of reason’s own possible forms. In his view the project of “critique”, his thorough examination of the limits of what we can know, had continually to wrestle with the mind’s tendency to overstep its own boundaries and, as Kant put it, to “rave with reason”, believing it could “see the infinite”. Rather than a delirium based on religious narrowness, Kant investigated fanaticism as a misuse of reason’s powers, an arrogant transgression of its own limits. Not parochialism and particularity, but something like an excess of universality defined Kant’s pathology of fanaticism. Hegel’s attempt to produce a philosophy of history – which Kant would have doubtless deemed fanatical in its claim to channel “Absolute Spirit” – also treated fanaticism, which Hegel recognised in both the expansion of Islam and the French Revolution, as a universalism, “an enthusiasm for the abstract”.

It was the French Revolution, with its unconditional affirmation of the rights of man against religious authority and social privilege, that did most to prompt alternative takes on what truly constituted “fanaticism”. The conservative reaction to the Revolution turned the tables on the Enlightenment, which was now attacked for its fanaticism, for its intransigent and destabilising political affirmation of the principles of freedom, equality and solidarity. Edmund Burke famously castigated this attempt to turn the principles of rational philosophy into the bases of civic constitution, arguing instead for the civilising function of hierarchy, religion, custom and gradual change against the ravages wrought by fanatical “political metaphysicians”. His Reflections on the Revolution in France established the template for subsequent pro-slavery attacks on Haitian revolutionaries and “abstractionist” American abolitionists (one of the few groups to reclaim the epithet “fanatic”), and filtered down to Cold War denunciations of the “political religion” of communism.

In the light of this historical legacy secularists and atheists face a predicament. Though vital in some situations, the simplistic opposition between tolerance and fanaticism can too easily turn into the defence of the status quo, and the defence of secularism into the revival of an imperial worldview that always treated legitimate resistance against it as irrational and deserving of repression. Conversely, an atheist politics will be met with cries of fanaticism from those liberals and conservatives who hold moderation and social peace, rather than social justice, as the chief principles of political administration. Only a fidelity to that Enlightenment which treats fanaticism – the refusal to compromise on certain principles and beliefs – as an intimate dimension of rationality, rather than its alien enemy, will allow us to confront these questions in the spirit of criticism and emancipation.

Fanaticism: On the Uses of an Idea by Alberto Toscano was published in June by Verso