For a long time, I have found that in many instances, what I draw is seemingly dictated to me by happenstance. So when I read a quote from Maggie Hambling, one of Britain's leading figurative artists, about subjects choosing her, it resonated! She said, "I believe the subject chooses the artist, not vice versa, and that subject must then be in charge during the act of drawing in order for the truth to be found. Eye and hand attempt to discover and produce those precise marks which will recreate what the heart feels. The challenge is to touch the subject, with all the desire of a lover."

Each draughtsman or woman knows that sudden rush of excitement when the eye alights on something that unpredictably seems so important, so much desired. It can be a wonderful pose by a model in life drawing, a tree standing proud and majestic in a field or park, an amazing landscape that calls out to be explored by drawing its features. Or it can be humble things, like feathers, bones, sticks or stones.

A vivid echo of this need to pick up tangible objects that represent landscapes and places that are really meaningful to an artist came today when I went to see the big Georgia O'Keeffe exhibition at the Tate Modern in London. A somewhat surprising exhibition in its lack of harmonious flow of content and awful lighting, there were, nonetheless, some of O'Keeffe's wonderful drawings from her early period. They are always inspiring and fascinating in their stripped-down power and distillation of material.

Apparently Georgia O'Keeffe was another artist who picked up leaves, stones, feathers, small disparate objects.

During her stays at Lake George, upper New York State, after 1918, she would go off alone in often perilously hostile weather on long excursions in the forests and mountains. Her "harvests" of potential drawings and paintings came back with her from these treks. They had told her to pick them up and take them home. Later, in New Mexico, she gathered stark, weathered bones that symbolisedthe complex, harsh environment of the desert. She said, "the bones are as beautiful as anything I know. To me they are strangely more living than the animals walking around."

So too, I find myself coming back from walks countless times with an odd collection of stones, bark, feathers - things that have suddenly called out to me to pick them up and keep them. They later "tell" me which to select for a drawing and then, how that metalpoint or graphite will evolve as "eye and heart attempt to discover and produce those precise marks that will recreate what the heart feels".

It is strange, mysterious stuff, this dialogue between subject and artist. I think it is a dialogue that evolves continually as the artist goes on in life. However, its power increases the more one respects this odd situation of the artist being merely the conduit for the subject of the artwork. Humbling, mysterious, yes - but endlessly interesting as one never knows what is going to happen!