In a book of essays entitled The Politics of Sex, Robert Grant, reader in English literature at Glasgow University, explores an astonishing range of subjects: Viz magazine, opera real, opera soap, Jane Austen, a self-styled freak show, sex, death, conservatism - and interior design. It is the last which particularly caught my eye. What could a renowned and controversial teacher of literature and recognised authority on the political philosopher Michael Oakeshott have to say about interior design? Quite a lot, actually. In an essay entitled Charles Rennie Mackintosh and the House Beautiful, he explores the meaning of interior design, and particularly of Mackintosh's world famous chair, or, as Grant puts it, ''that which makes martyrs of our backsides for the sake of art''.

Of Mackintosh's chair, he says: ''It is barbarism posing as hyper-sophistication'' and he compares it unfavourably with the ''civilised contrast'' - any mid-eighteenth-century chair - where ''flesh and spirit both find satisfaction in a perfect equilibrium''. His point is that our tastes reveal the society in which we live - our interiors are documents of political and cultural significance. This essay on the aesthetics of interior design is about the decline of taste, seen most clearly in the replacement of simple functionalism by the ''despotic ubiquity of meaning''. Behind all of this lurks a profound philosophical point.

Grant puts it thus: you can argue with someone about their reading matter, but to question taste in clothes or furniture is the height of impertinence. Why? Because one's inner self is much more closely implicated in them. Your furniture and such like are a kind of existential armour. Grant is picking up on an older tradition which asserts that someone's style, however inscrutable, is a concrete and revealing expression of the person themselves. In choosing what we desire to have as possessions, which includes styles in interior design, we display not just the design or object but also, and more importantly, oneself. The exercising of such choices is the manner in which human beings attempt to stamp their unique mark on the world and declare who they are. If this is true, when you attack my Ikea or Mackintosh chair, you attack me.

This is a familiar thesis in philosophy, often used to justify a number of claims, eg that human beings are possessive individuals, the importance and justification of private property, the importance of freedom of choice. To deny the latter in particular is to deny the individual personality its right to the freedom of expression.

The German philosopher Hegel, writing in the nineteenth century, is the main source for these and similar views. He maintained that the importance of private property lies in that what we gather around us is the very embodiment of our personality - an expression of the self. Indeed, private property could be described as a type of non-verbal language. And it is primarily through our property that others are able to recognise us as the beings we are.

This is vitally important for Hegel. For human beings, Hegel thought, only exist fully when they are recognised as human beings by other human beings. (No man in isolation is really a man.) Hence the desire for private property is not simply an expression of a bourgeois mentality, it is a means of achieving an existentially important goal.

It often looks, however, as though this manner of self-expression has gone too far and we have slid into decadence. It is not just that trophy consumerism is morally bankrupt, and ultimately not existentially satisfying. The problem now is that we have become too self-conscious in our display of our interior designs. We can no longer display something in innocence, without having to think about what we are saying or ''meaning'' by our displays, as everything we display must mean something significant about ourselves. This is the ''despotic ubiquity of meaning'' Grant so abhors.

The awful Changing Rooms is testimony to the need to express ourselves in this manner taken beyond the boundaries of endurance. And, as the queues along the A8 outside Ikea near Edinburgh demonstrate, interior design has become, in Grant's words, a kind of group therapy for those who cannot afford psychoanalysis. Things have got so bad that one might feel more at ease in Woolworth's or in an airport lounge. In these aimless concourses we are under no compulsion to select an identity, and can therefore simply be one's self.

And so we return to Mackintosh. In Mackintosh we find a designer so self-conscious and, in a sense so intent on self-expression, that the homely functionalism of a simple chair is abandoned in order to make an original aesthetic statement. But, as Grant says: ''Once the dyke of self-consciousness has been breached, we are all carried along in the ensuing flood.''