Davide Aldé

The thesis that the structure of experience is isomorphic to the structure of its physical correlate is central in the consciousness debate. It is arguably the most important proposed bridge between consciousness and the physical, and many in the field consider it key for identifying the physical correlates of consciousness and explaining the structure of experience. Yet this isomorphism is often assumed without clear motivation. I review the reasons behind this thesis and show that they all have important limitations. I then propose a transcendental argument for the existence of an isomorphism between experience and its physical correlates, grounded in the need to account for the structure exhibited in experiential reports. The argument also implies a constraint that can resolve a central difficulty: many physical structures are isomorphic to experience, making it hard to determine which count as physical correlates of consciousness. The constraint is that, among the physical structures isomorphic to experience, those that count as correlates are those causally responsible for the structural content of reports. This and other constraints can guide the search for the isomorphism and hence for the physical correlates of consciousness. The result is a clearer picture on what the isomorphism amounts to, why it obtains, and how to find and use it.

Structuralism, as I will use the term, is the view that phenomenal character is fully determined by the similarity relations between experiences, that is, that the way an experience of orange feels is entirely a matter of its relational profile within the space of color experiences. This theory has been criticized because it seems to leave out what is often referred to as the intrinsic character of experience. However, these critiques have failed to convince numerous scholars, largely due to their reliance on intuition, a tool that many consider unreliable. Here, I present two objections to structuralism that rest on firmer ground. The starting point is certain structural claims about color experience that are widely accepted in the science and philosophy of color. Then, playing the structuralist game through and through, I argue that the structure of color experience is incompatible with structuralism. I formulate two arguments to show this incompatibility: one focusing on unique hues (red, green, yellow, and blue), and a second focusing on the rule regulating how pure colors (red, green, yellow, blue, black, and white) combine to form composite colors. I consider some possible counterarguments, provide rebuttals, and conclude that structuralism should be rejected.