From his 1927 lectures published as Symbolism: Its Meaning and Effect.

While speaking about the way ordinary language can mislead us about the nature of reality, Whitehead begins reflecting on the common term “wall.”

“This so-called ‘wall,’ disclosed in the pure modes of presentational immediacy, contributes itself to our experience only under the guise of spatial extension, combined with spatial perspective, and combined with sense-data which in this example reduce to color alone. I say that the wall contributes itself under this guise, in preference to saying that it contributes these universal characters in combination. For the characters are combined by their exposition of one thing in a common world including ourselves, that one thing in which I call the ‘wall.’ Our perception is not confined to universal characters; we do not perceive disembodied color or disembodied extensiveness: we perceive the wall’s color and extensiveness. The experienced fact is ‘color away on the wall for us.’ Thus the color and the spatial perspective are abstract elements, characterizing the concrete way in which the wall enters into our experience. They are therefore relational elements between the ‘percipient at that moment,’ and that other equally actual entity, or set of entities, which we call the ‘wall at that moment.’ But the mere color and the mere spatial perspective are very abstract entities, because they are only arrived at by discarding the concrete relationship between the-wall-at-that-moment and the percipient-at-that-moment. This concrete relationship is a physical fact which may be very unessential to the wall and very essential to the percipient. The spatial relationship is equally essential both to wall and percipient: but the color side of the relationship is at that moment indifferent to the wall, though it is part of the make-up of the percipient. In this sense, and subject to their spatial relationship, contemporary events happen independently. I call this type of experience ‘presentational immediacy.’ It expresses how contemporary events are relevant to each other, and yet preserve a mutual independence. This relevance amid independence is the peculiar character of contemporaneousness. This presentational immediacy is only of importance is high-grade organisms, and is a physical fact which may, or may not, enter into consciousness. Such entry will depend on attention and on the activity of conceptual functioning, whereby physical experience and conceptual imagination are fused into knowledge” (p. 15-16).