Even if it cannot be defined as such, a thing has stable “characteristics” or “properties,” and we will approach the phenomenon of reality by studying perceptual constants. To begin with, a thing has its size and its shape beneath perspectival variations, which are merely apparent. We do not attribute these appearances to the object, they are an accident of our relations with the object and they do not concern the object itself. What do we mean by this, and on what basis, then, do we judge that a form or a size are the form and the size of the object?