When the political right confront the left in debate, the arguments of the former usually boil down to a simple underlying idea: that the left's "grand projects" of social change are incompatible with human nature. Those on the left, in this view, do not understand – or cannot bring themselves to accept – the grim reality in relation to the fundamental determinants of human behaviour. Human beings are essentially selfish, greedy, competitive, individualistic and generally unpleasant. This nature, furthermore, is fixed and immutable.

Rather conveniently, we happen to live in the kind of social order that is most in tune with our natural inclinations – a capitalist free market economy. In fact, for conservatives, capitalism is not really a discrete "system" at all; it is simply the natural and spontaneous state of things.

Almost all political ideologies, in fact, are based on a specific understanding of the nature of humanity, whether this is explicitly formulated or simply implied. The plurality of modern political ideologies implies a plurality of different conceptions of human nature – which implies, in turn, that the conservative understanding of human nature is contested. This is something that many conservatives have difficulty in accepting.

It is simply self-evident for most conservatives that human nature is unquestionably the way that they say it is (this is a defining feature of conservative thought in general, bound up with its broadly positivist assumption that one can understand the world through simple observation and application of "common sense") – and this tends to dictate that their conception of human nature is articulated at the level of mere assertion. Human nature just is egoistic, selfish and so on. Such an approach obscures the historical specificity of their view of human nature.

The modern right's understanding of human nature (and thus the broader political doctrine founded on this conception) first emerged with the birth of liberalism which was itself bound up with the emergence of capitalism. In fact modern conservatism is really a form of liberalism. The formation of the Conservative party in the 1830s represented an alliance of the rump of the old Tory party – previously dedicated to the defence of broadly feudal traditions and institutions – with a faction of the rising bourgeoisie dedicated to free trade and capitalist values. This marked the point at which conservatism, which used to be rather sniffy about nouveau riche bourgeois upstarts, reconciled itself to capitalism. Indeed the intimate connection between modern conservatism and liberalism is revealed in the political affiliation of Edmund Burke. Widely regarded as the "father of modern conservatism", Burke was in fact a Whig rather than a Tory.

Liberalism, which first emerged in the 17th century, has at its core a distinctive conception of human nature. The most important point about humans for liberals is the fact that they are individuals. It involves "seeing the individual as primary, as more 'real' or fundamental than human society and its institutions and structures" and "involves attaching a higher moral value to the individual than to society" (Arblaster). Furthermore, this conception of human nature "tends … to impute a high degree of completeness and self-sufficiency to the single human being, with the implication that separateness … is the fundamental, metaphysical human condition".

As a fundamentally "complete" individual, the liberal human has pre-given and fixed, rather than socially constructed needs and preferences. More often than not, the liberal individual is also a radical egoist who enters into interaction with other individuals simply in order to satisfy pre-formed preferences.

The relationship between this conception of human nature and capitalism is obvious. The atomised liberal individual reflects the atomised conditions of bourgeois society in which social ties of kinship and fealty have been dissolved. It is worth stressing that this was a new understanding of human nature. In pre-capitalist philosophy wholeness or completeness usually belonged to the community rather than to the individual.

Rather than self-sufficient individuals, humans were seen to be embedded in communal relations that almost wholly defined them. The view of human nature that underpins the politics of the modern-day right, then, arose at a particular historical juncture. It is not some ideologically "neutral" description.

So what, if anything, is human nature? Marx provides a much richer account. He is often said to have argued that there is no such thing as human nature. This is not true. Though he did think that human behaviour was deeply informed by social environment, this is not to say that human nature does not exist. In fact it is our capacity to adapt and transform in terms of social practices and behaviours that makes us distinctive as a species and in which our specifically human nature is to be located.

For Marx, we are essentially creative and producing beings. It is not just that we produce for our means of survival, it is also that we engage in creative and productive activity over and above what is strictly necessary for survival and find fulfilment in this activity. This activity is inherently social – most of what we produce is produced collectively in some sense or another. In opposition to the individualist basis of liberal thought, then, we are fundamentally social creatures.

Indeed, for Marx, human consciousness and thus our very notion of individual identity is collectively generated. We become consciously aware of ourselves as a discrete entity only through language – and language is inherently inter-subjective; it is a social practice. What we think – including what we think about ourselves – is governed by what we do and what we do is always done socially and collectively. It is for this reason that Marx refers to our "species-being" – what we are can only be understood properly in social terms because what we are is a property and function of the human species as a whole.

Marx, then, has a fairly expansive view of human nature – it is in our nature to be creatively adaptable and for our understanding of what is normal in terms of behaviour to be shaped by the social relations around us. This is not to say that any social system is as preferable as any other. We are best able to flourish in conditions that allow us to express our sociability and creativity.

As Terry Eagleton argues, Marx's notion of "species-being" implies an ethics of self-realisation or flourishing through social interaction. Eagleton argues that Marx was "a closet Aristotelian of sorts", by which he means that Marx, like Aristotle, felt that humans live well when they act to realise their own creative nature. The main difference between Marx's (tacit) ethics and those of Aristotle is that for Marx, self-realisation must be an reciprocal process because we are social animals.

"What this means", Eagleton explains, "is that we become the occasion for each other's self-realisation. It is only through being the means of your self-fulfilment that I can attain my own, and vice versa." "The political form of this ethic," he continues, "is known as socialism", which can be regarded as a form of "politicised love, or reciprocity all round".