Knock on Wood (2020) presents Robert Irwin's perceptual investigation at its most distilled, engaging the viewer in a sustained encounter with light, color, and the subtle physics of presence. Spanning nearly three meters in width, the work operates through layered translucency, casting shadows and reflecting ambient light in ways that shift continuously with the conditions of a given space. The piece does not simply occupy a wall; it activates the surrounding environment, dissolving the conventional boundary between object and atmosphere. Color here is not applied so much as induced, emerging from the interaction of materials and illumination rather than from pigment alone. Irwin has spent decades developing a practice rooted in phenomenological inquiry, and Knock on Wood distills that project into a work of remarkable economy and depth. The title carries a colloquial warmth that sits in productive tension with the work's formal rigor, suggesting the artist's ongoing interest in bridging sensory experience with something more intuitive and bodily. At 182.9 by 276.2 centimeters, the piece commands the scale of a significant painting while resisting the logic of the picture plane entirely. Its shallow depth of just under eleven centimeters belies the spatial generosity it creates in installation. Offered through Sprüth Magers, this work represents a meaningful opportunity to acquire a late example from one of the foundational figures of Light and Space, a movement whose influence on contemporary installation and perceptual art continues to grow in critical and institutional recognition. The absence of a frame is not an oversight but a condition, ensuring that the work remains open to its environment rather than sealed against it.