Who was Giancarlo De Carlo, who died on Saturday 4th June, aged 85? He was both elusive and yet absolutely clear. He was both renowned and yet a secret. One of the foremost architectural thinkers of his time, he published no unified volume of theory. He was not an architect who played at being a theorist, but an intellectual whose medium was architecture. (That is, not abstract writing around architecture, but its concrete profession, embedded in its social practice.) He was one of the most memorable architectural teachers of his generation, and yet always set himself outside, at a critical angle to the academy. He enjoyed disguise, but never attempted to hide. He wrote under transparent pseudonyms – such as Heres Jedece or Ismé (only understood when pronounced as if it is English). His career seemed as a resistance movement born in the interstices of modernism; yet he was never in the shadows.

He was, as his close friend Aldo van Eyck first said half a century ago, a master of paradox. With infuriatingly paradoxical consistency, his evolving and often unexpected architectural language, over more than half a century, ever thwarted our wish to pigeon-hole. Yet his themes, always modulated by contingent reality, have remained stoutly consistent. If De Carlo was, as Manfredo Tafuri suggested, a rare intellectual in architecture, he was scathing of architectural theorists, his aim always being action. And his rigorously achieved socio-political position found expression in over half a century of coherent work: writing, teaching and publishing, design projects and planning studies; and, centrally, interventions in the built and inhabited fabric of our world.

By teaching that architecture cannot be dissociated from the social and moral conditions of the age to which it belongs, he restored to the architect an awareness of his mission among humanity. With his work and the example of his life, he showed that anyone who wanted to build for people had to share in humanity's problems and misfortunes, to struggle at its side in order to satisfy its moral and material needs. He taught that, if architecture were to be authentic, it could not be limited to a question of taste or style, but had to expand, to become an active principle that took in all human activity.

That last paragraph is how, nearly 60 years ago, Giancarlo De Carlo described William Morris. The world, emerging deeply shocked from the Second World War, was then at a point of immense potential. De Carlo enlisted Morris in his cause, adding that his teaching formed the ethical foundation of modern architecture, the foundation that links modern architecture to the story of the struggle for human freedom. While it might equally stand to describe De Carlo himself today, it is not a media-friendly message. De Carlo is not easily consumed: his words are carefully chosen, his arguments subtle. While his space-making is highly sophisticated, his architecture does not consist of a series of objects which can be captured on the pages of a glossy picture book. Rather it is a flux, it is slippery, elusive and not easily pinned down; it consists of processes rather than products, means rather than ends; it is centred on people and action; and it is meaningless without them.

 
 

The career of De Carlo, born in December 1919, offers an original lens through which to focus the last half century's architectural debate; while it also articulates a role for the committed professional actor today. How was he located in these currents? Discussing the post Second World War years, key architectural themes were recently listed (by S. W. Goldhagen in Anxious Modernisms) as: popular culture and everyday life, anti-architecture, democratic freedom, homo ludens, primitivism, authenticity, architecture's history, regionalism and place. Giancarlo De Carlo was at the centre of exploration of each one of these issues.

And of the 1970s? That was the time, Tafuri suggests, when "the most interesting designs have given rise to themes resolving around the concepts of place, context, modification, reweaving, the relationship between an intervention and its surround conditions...". Which is exactly what De Carlo had always been up to, not least when it was far less fashionable than it later became.

And then the key themes of the 1980s? Frampton's construction suggests four important groupings: the dominant Populism and Productionism (that is, broadly, post-modernism and high-tech), the critical Rationalism and Structuralism (the former, a "desperate artistic commentary on the lost nature of the modern city", led by Aldo Rossi and Giorgio Grassi, the latter named from structural anthropology was led by Aldo van Eyck and Herman Hertzberger). He then suggests that a further critical strain would lead architecture forwards; it would have a commitment to place and "tectonic density"; he called it regionalist. While Frampton's final images were of Piccionis' hill-path in Athens and Utzon's Bagsvaard church, it is De Carlo who has come as close as anyone to resembling Frampton's critical regionalist.

And of the later Twentieth Century? A more pessimistic view of its architecture sees it as oscillating between the yearning for a technocratic order and an idiosyncratic individualism. This theme, articulated earlier by Tafuri and Francesco Dal Co among others, is taken up by Dalibor Vesely: "The technical homogenisation of modern life makes it much easier to share the illusion that even the most abstract architectural solutions, based on narrow technical criteria, may be adequate and appropriate. ... The anonymity and the disembodied nature of modern technology is complemented by a second main modern tendency, represented by an introverted and highly personalised culture. ...the architect sees himself as a sole agent fully responsible for everything related to creativity. ... The concentration on private experience, imagination and fantasy contradicts the very nature of architecture, which is always open to and judged by a shared public culture." It is against precisely these trends that De Carlo has stood; and it is in what Vesely calls that 'grey zone of modernity', lying between the solipsistic world of personal experience and an abstract culture dominated by the instrumental, that he has always attempted to operate.

De Carlo would argue that the space assigned to the architect has nothing innocent about it: it answers to particular tactics and strategies; it is, quite simply, the space of the dominant mode of production, and hence the space of capitalism governed by the bourgeoisie. These words, however, are from Henri Lefebvre's The Production of Space, alongside which De Carlo's critique of architectural practice stands close.

Lefebvre's criticism is of architects' complicity; in their narrow privileging of the image and of its consequence, an impoverished understanding of inhabited space. He attacks the architectural discourse for its imitation or caricature of the discourse of power; suffering from the delusion that 'objective' knowledge and 'reality' can be contained in its graphic representations, a discourse wherein the user's space find expression only with great difficulty. De Carlo, who had already been arguing very similarly, was teaching in the Venice architecture school in May 1968, at the hub of the Italian revolt; while Lefebvre, in Nanterre, was a key influence behind the Parisian students. They linked in the 1970s when Lefebvre's briefly flourishing magazine Espaces et Sociétés became transformed into De Carlo's Spazio e Società, a magazine which he ran until its end in 2001.

 
 

While he was always building, writing essays and lecturing, since the 1970s De Carlo made a quiet stir with his own independent platforms for debate. Both Spazio e Società and the remarkable master class, the International Laboratory of Architecture and Urban Design (ILAUD) meeting annually in Urbino, Siena, San Marino and now Venice, were entirely De Carlo's children. Buttressed by Team 10 and other younger colleagues from around the world, they calmly rode through tides of the 'tendenza' and 'post-modernism', of 'community architecture' and 'vernacularism', of high-tech and the signed artwork. But this should not suggest that his ideas or his own expressive language stood still.

De Carlo's understanding of the heritage of the historic city and the debate as to how it can be renewed, always avoided nostalgia – from post-war neo-realism to post-modern neo-historicism. What his work teaches is the remarkable value of a participatory, deep reading of place; its specificity in formal fact, its material and social formation buried in civic memory, its present social inhabitation and aspirations, its offering links to possible, but unknown, futures. His own language evolved and developed extraordinarily, particularly in the decades when more sensible men were enjoying retirement: in many ways his most significant built oeuvre dates from the years since his 65th birthday. HJ was busily designing housing in Beirut, though frustrated by being too ill to do much, when we last met a few months ago.

His buildings are complex responses to particular sets of questions, and do not reveal themselves all at once. De Carlo does not feed art editors' desire for abstract art-work in the landscape. His drawings have little intrinsic value beyond their part in the more important production process. Meaning is breathed into his spaces by honouring the complex processes of their formation and their creative inhabitation. As Hertzberger quoted at one of De Carlo's ILAUD meetings, "Wie die Räume ohne Menschen aussehen, ist gleichgütlig. Wichtig ist nur, wie die Menschen darin aussehen." It doesn't matter how it looks without people. What counts is what the people in it look like.

De Carlo's ideas were also rooted in practice; but that in no way implied a simple pragmatism. Theory and practice are not autonomous, architectural hypotheses are inevitably social; architecture without building is nonsense, as is building without architecture. His architecture centres on buildings - needed, created and occupied – whose enclosure and space is only the outline of a potential made relevant by the group of people it is intended for. For him "the reality of a building consists in creating a congenial condition in which a society, using that building, can make choices and mix together."

He never dealt with a 'how?' question without considering the underlying 'why?' This did not endear him to authorities (who resent being asked why their housing budget is so parsimonious) or to colleagues (whose sails are trimmed to fit prevailing political winds). All fine architects' careers are strewn with disappointments; competitions lost, projects foundered, client's lacking courageous commitment. De Carlo, however, by refusing to temporise and – uniquely in Twentieth Century Italian culture – by refusing to align himself with the essential channels of political patronage, ensured that his output remained even smaller than most. In Milan, his home city, for half a century he said he had not even been asked to produce a dog-kennel. It was then particularly poignant to see him receive an honorary doctorate from Milan just before last Christmas. The spontaneous standing ovation that greeted his entrance in wheelchair, lasting for minutes, may have been tinted with tinges of guilt. His grateful acceptance speech was as sharp and aware as ever of the ironies. Perhaps his Italian reputation is reclaimed: he died the very day a major exhibition of his work opened in Rome.

His reputation, therefore has had an unusual, and elusive quality. Italians seemed to remember his contribution to debate in 1950s in Casabella, then Team 10, the Urbino Collegi and perhaps his Terni housing - then silence (at least until the odd sharp essay in recent years). Spazio e Società, though co-produced with MIT for some years, was hardly noticed at home. He had become isolated both in the profession and in the academy; with his culture always antithetical to provincialism and to fashion, with his materialism untouched by the Italian love of ambiguous metaphysics. It is partly the perception, exemplified by Tafuri, of a clear generational gap between De Carlo, Gardella and others on the one side, Gregotti, Aldo Rossi and their friends on the other. But it is not an issue of age. Was his really a generation of architects doomed, as Joan Ockman suggests, to the limbo between modernism and postmodernismWhat then did it mean for De Carlo to embrace modernism?

 
 

In most of the west, pre-war architectural modernism had been the province of an avant-garde minority, where it hadn't been repressed by the Nazis. In Fascist Italy, however, it had thrived with an ambiguous difficulty. Then, following the war, it quickly become the face of institutional capitalism, becoming seen to serve other repressive ends, while to the avant-garde its monolithic International Style now seemed arthritic and rigid. De Carlo's whole career, starting up at that moment, could be seen as an interrogation of modernism; but what was the image of modernism at the core of his critique?

"All that is solid melts into air, all that is holy is profaned, and men at last are forced to face with sober senses the real condition of their lives and their relations with their fellow men." These famous lines at the core of The Communist Manifesto define a modernism De Carlo would certainly recognise. Marshall Berman's take on modernity slams the city of Le Corbusier, Siegfried Giedion and CIAM as "a modernised vision of pastoral: a spatially and socially segmented world... where haloes could begin to grow around people's heads once again." In contrast, with more enthusiasm, Berman embraces today's "contemporary desire for a city that is openly troubled but intensely alive, that is a desire to open up old but distinctly modern wounds once more. It is a desire to live openly with the split and unreconciled character of our lives and to draw energy from our inner struggles, wherever they may lead us in the end. If we learned through one modernism to construct haloes around our spaces and ourselves, we can learn from another modernism to lose our haloes and find ourselves anew."

Here is the modern world within which Giancarlo De Carlo worked. Modernism died, will die many times and, he argued, it will be reborn; "because, under its ashes, the matrix that has generated its previous existences remained intact: This was the principle that architecture, if it is to find again its most authentic essence, must be disentangled from the requirements of power." Such an architectural production cannot avoid conflict. We must exploit the uses of disorder, and run the risks inherent in that undertaking. Fluidity is of its essence, openness to the changing situation. The city, as one organism rather then an aggregation of objects, is swirling, gathering at points he calls (and designs as) 'condensers'. Recently, for a major project in 2001 (a final stage competition against Koolhaas and van Berkel), sensing the time for his ideas to be ripe, he argued: "the idea is in the air that the 'project' that aims at the 'stable' and 'finite' no longer corresponds to the state of things, and that the only way to organise and give form and space is to start 'processes'..."

As a practitioner's approach to urban planning it has been little understood. Does this contribute to how De Carlo managed to remain so much a secret? While, for example, Lefebvre's critique was growing in influence, so De Carlo was attempting to approach a similar agenda from the position of engagement. His muddy realism of conflict and commitment was a long way also from the nihilistic ivory tower from which Venice's famous architectural historians could sneer. There is irony here in how theories of architecture as event and situation can transfer into a context of critical theory or of art production, while their architectural manifestation might pass unrecognised. While Lefebvre theory becomes required reading among thinkers about the environment, De Carlo's praxis becomes marginalised today among the tasty flavours enjoyed by teachers and publishers of young architects, and so almost invisible. Theory and practice are comprehended separately; and where they are intimately connected, we tend to look at the wrong thing, at images rather than events.

The Penguin Dictionary of Architecture, but only in its Italian edition, described De Carlo in 1981 as "one of the most interesting post-war Italian architects," yet he seemed better known abroad, being a regular visiting Professor in the United States, then recipient of the UK's Royal Gold Medal for Architecture. When the AJ poll in 2002 put De Carlo eighth in a list of the most influential architect in the world alive today, where might an Italian poll have ranked him? In Urbino, a city to which he devoted his life, he appeared Milanese, to Milan he was oddly Anglophile (and his studio always used to stop for tea at five o'clock in the afternoon); while to the British and Americans he remained very Italian. Perhaps, finally, he was happiest on the margins, in the interstices, always resisting the thoughtless flow. "I have always been a outsider [he uses the English word 'outsider' in this Italian conversation]. I taught at the university for a long time but have never been an academic; I had a practice of design for many years but never became a real professional; I write about architecture but am not a writer; I draw and paint but I don't sell my artwork. At the end of the day I simply keep alive the energy from my childhood."

"So why is it that after being famous, he is now almost unknown? Probably because he is intrinsically an anti-establishment character, he does not belong in the dominant neo-capitalist and consumption-oriented flood that is enervating the West; because he remains an outsider in the academic world; because he has harshly criticised the blindness of Modernism without deigning a glance at the idiocy of the Post-modernists; and finally, because he has not made a pact with the devil of the mass media ..."

Here was Giancarlo De Carlo talking, in 1990, about Christopher Alexander. But fully aware that it was also a canny and proud self-portrait.