This evocative quote from John Berger's Here Is Where We Meet: A Fiction captures a profound meditation on the nature of perception, aspiration, and possibly the illusory constructs we build around our understanding of the world. The phrase "all its dimensions with their projected geometries" hints at a complex, multi-layered reality—one that is mapped and shaped by mental projections, interpretations, and expectations. Yet, these geometries are not firmly planted in tangible truth but exist within the realm of an "unrealisable dream." This sparks a reflection on how much of what we conceive remains intangible, idealistic, or even fundamentally unattainable.

On a metaphorical level, the passage suggests the tension between what is imagined and what can be brought into existence. Dreams, no matter how vividly envisioned or meticulously designed, often elude realisation due to constraints beyond the dreamer’s control. Such a perception can evoke both melancholy and a haunting beauty—a recognition that some dimensions and forms of understanding may forever belong to an ideal plane rather than to mutable reality.

Moreover, Berger’s use of "projected geometries" elegantly connects to the theme of perspective. Geometries are precise, mathematical; projections imply intentionality, a shifting of reality onto a conceptual framework. This, therefore, might reflect on human experience itself: life as a series of projections, shapes that might not fit perfectly in the real world but shape our conscious sensation of it nonetheless. In essence, the quote circles around the paradoxical beauty and sorrow of human aspirations: our dreams shape reality in thought, yet often remain just beyond our grasping.