‘Behind every man's external life, which he leads in company’, the mid- Victorian journalist and economic adviser Walter Bagehot observed, ‘there is another which he leads alone, and which he carries with him apart’ (1965a: 195). To further characterise the distinction between the individual's innermost private space and the outer world of sociality, Bagehot formulated one of his many trademark quips: ‘We all come down to dinner, but each has a room to himself’ (195). In his evocative phrasing, which locates in the deep recesses of the mind the privacy that secures individuality, Bagehot employs a metaphor that refers to one domain of experience (thinking) in terms of another domain (space). He was not alone in conceptualising the mind in these terms.

For the intellectual heirs of John Stuart Mill, Bagehot among them, spatial metaphors were a way of defining liberal individuality as founded in a possessive relation to mental privacy. In classical liberalism this possessiveness had been presumed to originate in ownership of private property. The liberal subject emerged in a private sphere of intimacy (the home) before joining the public sphere of civil society to debate in a disinterested manner various matters of general interest. Property ownership, because it freed one from either patronage or influence, ostensibly facilitated the ability to detach from self-interest when participating in these deliberative exchanges. But Mill redefined this relation to privacy. To the extent that one had recourse to an inward space of thought and reflection, one possessed the capacity to produce in private the individuated, liberalised ideas that would be expressed publicly as opinion.

Mill had not been raised to valorise individuated privacy. Instead, from an early age, he was shaped as a proponent of utilitarianism, a school of thought that originated with Jeremy Bentham, a family friend. He was also trained as a subscriber to the theory of associationism promulgated by his father, James Mill. Associationist psychology assumed that the building blocks of an adult's cognitive architecture were the mental impressions – rudimentary ideas or images – generated from sensory data and perceptual stimuli experienced in childhood. In the elder Mill's view, when combined through the association of recurrence, simple ideas formed more complex ideas and complex ideas formed duplex ideas.