The paintings and drawings by Jo Baer, Mary Corse, and Agnes Martin in “At the Edge of Things” range in date from 1959 to this year, but all are grounded in phenomenology and investigations of borders. Corse has beveled some of her stretcher bars so that the paintings become wall-mounted plinths, extending out to meet the viewer’s gaze. Most of Martin’s vertical graphite lines in Untitled #12, 2002, do not touch the edges of the canvas but fade out near the top and bottom. An arm span of taut cloth becomes an arena for mental projection, as evinced by occasional titles (Gratitude, 2001, and Aspiration, 1960, for example).