In the great spat between King Kong Chomsky and Tyrannosaurus Žižek people are often asked which side they are on. Or maybe they are not, because until now these two great beasts have been roaring and knocking down trees without anyone outside leftist discourse hearing them fall. But maybe we should think who we would cheer on, because this is a debate about something very important – namely the relationship between theory, ideology and reality.

Noam Chomsky, the professional contrarian, has accused Slavoj Žižek, the professional heretic, of posturing in the place of theory. This is an accusation often levelled at Žižek from within the Anglo-Saxon empirical tradition. Even those like Chomsky who are on the proto-anarchist left of this tradition like to maintain that their theories are empirically verifiable and rooted in reality.

Žižek has countered with the side-swipe that nobody had been so empirically wrong throughout his life as Chomsky. He brought up Chomsky's supposed support for the Khmer Rouge in the 1970s and Chomsky's later self-justification that there hadn't been empirical evidence at the time of the crimes of the Khmer Rouge. It has all got rather heated and intemperate, but then, debates on the left are like that. More time is spent ripping flesh out of each other than it is trying to find a common cause against an apparently invisible and impregnable enemy. But terms have to be defined, ground has to be laid out.

Chomsky is also probably still smarting from his encounter with Michel Foucault in 1971, on questions of human nature versus socialisation. Foucault argued that human society produced ideas in individuals which were the product of the power relationship between those individuals and society. In Foucault's view society took precedence and individuals are unable to uncouple themselves from the power relations at play and which soaked through everything. In which case, it is necessary to have a speculative theory about how the relations of power might work in psychoanalytical terms. This is part of a long tradition of Ideologiekritik.

Žižek stands in this same continental tradition (as well as against it, but, hey, that's his job) of asking ontological questions – that is, questions about being as an abstraction – rather than trying to find out through supposedly scientific methods what human nature actually is. There is an old joke that goes "the Anglo-Saxon philosopher will accuse the continental of being insufficiently clear, while the continental philosopher accuses the Anglo-Saxon of being insufficiently." For Žižek there is no finished human nature, but rather simply a process of working out how human beings are in the world. At the core of this argument is a question of whether the word "real" is spelled with a capital letter or not.

For the empiricists the word "real" refers to something, well, real; something pre-existing which has to be uncovered. For the Žižek/Lacan tradition, the word is spelled "Real" and refers to something which isn't real, is inaccessible, and which can never be defined as it is still, with Hegel, "im werden" (or, in becoming). This "Big Other", as Lacan termed it, is the hole occupied by the absent father or God, so that the Real is only present through its absence. This sort of stuff is dismissed as charlatanry by those who want something concrete to hold on to, whereas for the continentals the hole was always part of the whole. Our being is conditioned by absence, by the something that is missing and by the desire to fill that gap.

Of course, when people look back from the future they will probably laugh about these debates and perhaps wonder whether they were the reason great beasts like this eventually went extinct; but to paraphrase Bertolt Brecht, somebody has to do the dirty work so that "those who come later" can look back and laugh.

• This article was amended on 19 July 2013. It originally referred to Chomsky's support for the Khmer Rouge in the 1960s. 1960s has been corrected to 1970s and the qualifier "supposed" has been added