The recent Equality and Human Rights Commission (EHRC) report, How Fair is Britain?, has elicited a flurry of criticism from those who regard egalitarian commitment as a misguided, if not sinister, political approach. In his Telegraph blog, for example, the impressively rightwing Ed West thunders against the EHRC and "equality of outcome" which, as every Telegraph journalist knows, is a Bad Thing and, anyway, "impossible".

In this newspaper, Julian Glover informs the left that it should give up on the ideal of equality since it is "undesirable". In the Independent, Dominic Lawson asserts that demands for equality and fairness are motivated by "envy" and, as such, should be dismissed. All these objections draw on the familiar anti-egalitarian arguments regularly deployed by the right. Many egalitarians might be tempted to shrug them off as the usual arguments from the usual suspects. But, for me, there is something deeply irritating about them. It's not just that I disagree – it's that these arguments are founded on caricature.

All of these commentators assume that when the left talks about equality it means absolute equality of everything. This is a common assumption among those hostile to egalitarianism: that the left want everyone to be exactly the same. No serious political theorist, however, has ever argued for such a self-evidently absurd position. Marx certainly didn't.

Indeed, Marx's summary of the principles that we would/should obtain under communism – "from each according to his ability, to each according to his need" – implies, precisely, significant inequalities in the distribution of social goods and also rests on the assumption that abilities are unequally distributed, too. Indeed, one of the things that Marx is trying to show is that for a rough "equality of condition" to be obtained, inequalities between individuals are necessary – since we all require and desire different things in different proportions in order to flourish. To treat people as if they are exactly the same is, in fact, to treat them unequally. Equality for the left is a complex concept, which bears little resemblance to the caricatures drawn by the right.

Martin O'Neill brings out something of equality's complexity and usefully draws our attention to the distinction between two conceptions of equality often conflated by the right – equality of opportunity and equality of outcome. The former focuses on the equalisation of opportunity for those with the requisite capacities or abilities to obtain a particular advantaged social position – it focuses on the elimination of arbitrary discrimination in the process of selection for such positions and, by definition, justifies certain inequalities of outcome. This, by the way, is what Glover appears to favour – he's not actually against equality per se, but against a certain (caricatured) version of equality (of outcome).

In fact, it's hard, today, to find anyone who is really against equality. The political dispute in relation to equality is not between egalitarians and anti-egalitarians. It's a struggle over the definition of the principle of equality: what is to be equalised, between whom equality should be obtained and where the limits should be drawn. Commitment to the notion of equality is deeply embedded in the fabric of modern politics.

Along with liberty, equality was one of the two key principles that drove forward the various 17th- and 18th-century revolutions (English, French and American) that inaugurated the liberal world. The modern liberal and capitalist order, then, has revolutionary beginnings (something that, as Terry Eagleton points out, liberals today find acutely embarrassing and try not to mention), but with the establishment and gradual consolidation of capitalism and liberalism, the tenor of liberal ideology shifted from one of radical optimism to one of "moderate", "realism" and scepticism towards grand projects of social change.

As the philosopher Étienne Balibar has argued, however, (see also Alex Callinicos), liberalism's revolutionary ideals retain an inherently subversive nature. Balibar argues that the universalism of core liberal principles – equality and liberty – imbues them with a radical logic that tends to come into conflict with the structural foundations of bourgeois society.

The bourgeois revolutionaries overthrew the ancien régime in the name of freedom and equality for all – and though these ideals were "packed with tacit or explicit clauses excluding women, the poor, slaves and many other groups from its ambit" (Callinicos, Equality), the universalism of these ideals had a tendency to breach these morally arbitrary, power-determined limiting clauses. Groups who were excluded from the realm of liberal equality and freedom (slaves, women, workers) could draw on the stated or implicit universalism of these liberal principles and demand inclusion. Each rectification of injustice drew attention to further forms of injustice so that the extension of liberty and equality moved forward in a rolling, cumulative progression.

For socialists, the next stage in this process of extension is a struggle for economic and class equality. Inequalities of wealth and economic power entail, after all, inequalities in political power. Why doesn't the ambit of democracy – equal liberty amongst citizens to exert control over social processes – extend into the economic sphere? Doesn't equality (and liberty), socialists ask, require economic democracy? This, however, would be incompatible with capitalism. Beyond a certain point, then, the dialectic of struggle for the extension of liberal ideals of liberty and equality becomes a definitely socialist struggle.

One can see something of this tendency of liberal ideals to go beyond themselves in the way in which equality of opportunity, on close inspection, slides into equality of outcome. Equality of opportunity requires that each individual has an equal starting point in competition for particular social goods – outcomes reflect ability and effort. The problem is that outcomes are also starting points. A child's starting point, for example, might be to be born into an affluent family – but this is the outcome of the parents having successfully made use of their opportunities. This suggests that if we really think it's important to equalise opportunities we need to equalise outcomes too.

We need, also, to question what counts as morally arbitrary criteria in the equality of opportunity view. There is no logical reason, in terms of justice, why if it's wrong to discriminate against people on the grounds of race or gender, it's not also wrong to discriminate on the grounds of ability or intelligence. Does an intelligent individual deserve higher rewards simply because they are bright? Why? Surely, they have no control over this any more than they have over their sex or skin colour. This doesn't mean that ability ought to be irrelevant in allocation of jobs – nobody wants to be treated by a brain-damaged brain surgeon. It does, however, suggest that there is no good reason why higher rewards should be distributed on the basis of these criteria.

The logic of liberal thinking on equality and justice always points towards equality of condition and, since it is difficult to see how such radical equality is compatible with capitalist relations of power, the logic of liberal thinking points beyond itself, towards socialism. This, by the way, is something in relation to which liberal political philosophers expend an extraordinary amount of effort pretending not to have noticed.

• Ed Rooksby posts on Cif as RedMutley