Mental "structures" in the Berlin school of Gestalt Psychology: can sensation be described as "structural"?

It is not exaggerated to affirm that the modern notion of structure arises in Koffka’s Growth of the Mind and in his following article, “Perception : An introduction to the Gestalt-theorie” (1922). The importance of the notion of structure as Koffka uses it lies in the fact that it is designed to replace the old empiricist notion of “sensation” as a real and separable element of the phenomenal field, corresponding to a definite stimulus. But, yielding to many suggestions by Köhler, Koffka does not only understand the interdependency of sensations in a structure as a causal one: in fact, he decidedly understands it as a logical one. Thus he defines structures as “very elementary reactions, which phenomenally are not composed of constituent elements, their members being what they are by virtue of their ‘member-character,’ their place in the whole; their essential nature being derived from the whole whose members they are” (“Perception”, p.543). I mean to show that the parts in such structures can only be what it is classical to name “relational attributes” or “relational predicates”. In other words, structures are now internal relations between their terms, and more precisely still “directly constitutive internal relations”, not internal relations reducing to the existence of their terms as were the internal relations against which Russell struggled, but relations to which their terms reduce. But the real importance of this notion of structure is that it rests and is built upon a truly impressive amount of empirical data. Nevertheless, I want to show that Koffka’s conception of sensation is fundamentally impossible to conceive, and that the belief that it is empirically grounded rests mainly on a confusion between abstraction of a sense-datum and real separation of the stimuli underlying such a datum. As a consequence, phenomenal structures, if they exist, can only be external to their terms, as they are in Köhler’s view, in spite of many ambiguities in his formulations. However, I will end by showing that, correctly understood, the notion of structure can still be of great help in phenomenology and psychology since it provides a naturalistic means to understand how a non-intentional “meaning” can be passively present at a sensory level.