Henri Matisse made "Blue Dress Reflected in a Mirror" using an expressive, process-led approach to painting. Look at the planes of colour, how each area is filled with strokes. It's like he's building up the image, brick by brick. The materiality of the paint is so important here. It's thinned, but still so present, so gestural, that you feel the movement in the brushstrokes. See the way Matisse uses that thin white line to describe the frills of the dress? How that line dances over the dark blue, creating volume and depth with just a few marks? It feels like a record of a thought, a moment. Matisse is an interesting artist, his interest in colour and mark-making is really unusual. He often gets compared to Picasso, who also pushed the boundaries of representation, but Matisse goes further somehow. He finds something else out there, beyond likeness. He shows us that art is a conversation, always asking questions, and never giving fixed answers.