I suppose some little boys and girls will be scratching out earnest epistles to Santa as I speak. Maybe something along the lines of: “I’ve been ever so good this year, helping Mummy with the baby and hardly hitting brother at all. Please can I have these things I have written and I promise to be good for all of next year: a pony, a dolls house, a box of colouring crayons and a bike.”

And as all those hundreds of thousands of envelopes get plastered shut, stamped and posted with care, it’s a letter of another order I’m delivering to you today. This one’s by the American Minimalist Robert Morris (born 1931), who I’ve grown to like a lot but who leaves me a bit baffled and befuddled at the best of times…

It was involvement with avant-garde theatre that inspired Morris’s classic 1960s Minimalist works. It seems to have been a small and simple step: he’d used basic geometric constructions as props in his performance pieces and thus spied their potential as sculptures in their own right. Doing things like placing large plywood polyhedrons in a gallery led to what Morris called an “extended situation” which was all about the ‘embodiment’ of the viewer: rather than peering in close to a piece of his art and pondering the construction and composition, Morris moves the viewer to become aware of their relationship to the objects in space and time.

This larger-scale spatial exploration that Morris maneuvers with big pieces is all well and good (and indeed it had a profound influence on contemporary art) but how are we to approach a much smaller work like Untitled (The Letter), 1964? This is painted lead and locked in place onto the wall in the concourse gallery of the NGA East.

For a start it’s more figurative than I’ve seen from Morris: a little lead letter suspended on two fine filaments before a slate grey ledge and a panel of white. It fits with the Minimalist mantra that a work is neither painting nor sculpture, but beyond that, what can it mean?

For me in any case Morris masters a subtle and quite sublime sense of tension here: the unopened missive must be one of the most simple and yet totally tantalizing subjects he  could have picked. And I suppose it does do what his overall oeuvre aims for, i.e. getting us to contemplate our relationship to the piece in the context of space. With this letter there unfolds a lovely layering of space, between the envelope, the panels, the wall and us. Because we move, it’s a fine and fragile state of flux that Morris manages to make us see.

But of course, all we really care about is what’s written in that letter.