N ico O rlandi While hallucinations are experiences of some kind, perceptions are relational states that are partly constituted by their objects When the objects are not present, perception does not occur Thus, while the representationalist thinks of perception as essentially a relation to representations, the disjunctivist thinks of perception as being fundamentally a relation to objects (Campbell 2002).11 For this and other reasons, John McDowell has argued that disjunctivism is the only theory that explains how perceiving agents are in genuine contact with the world Other theories, according to McDowell, are all too friendly to skepticism M.G.F Martin has similarly argued that disjunctivism is the only theory that accords with our common sense view of perception Disjunctivism respects the common sense idea that perception is a type of ‘openness to the world’ In sum, if we conceive of perception as a relation, we can see the 20th century in philosophy as a struggle to defend realism and physicalism Overall, indirect realism is more common in the first half of the century, while direct realism is preferred in the second half, with theories battling over which does a better job at rescuing direct realism Physicalism is also a constant concern, especially in the second part of the century In the next section, we consider perception as a type of unconscious process 2. The perceptual process The philosophy of perception in the last century has been shaped by cooperation with the developing field of psychology Psychological theories have in turn been influenced by work in philosophy The interplay between these and other disciplines eventually developed into what is now known as ‘cognitive science’ Cognitive science is a collaboration of academic disciplines geared at understanding mental states and processes It is controversial whether this is a unified field, but it includes psychology, philosophy, computer science, linguistics and neuroscience Cognitive scientists tend to be interested in perception as a process, and to focus on the problem of understanding how it is that we come to perceive the world as we (Palmer 1999) Perception starts with the stimulation of sensory receptors by environmental elements – for example, by light hitting the retina or by sound waves affecting the ears’ membranes The question is how, from this sensory basis, we come to perceive distal objects and events The major psychological theories of perception in the 20th century that address this question are either reactions to, or continuations of what happened in the century before The second half of the 1800 saw two important developments in the new field of perceptual psychology First, Wilhelm Wundt proposed structuralism, a position inspired by the British Empiricists Structuralism was later popularized in the United States by one of Wundt’s students, Edward Titchener and, as we will see, it is the psychological counterpart of sense-data theory (Titchener 1902; Wundt 1874) Second, Hermann von Helmholtz – a German physicist, mathematician and physiologist, who also supervised Wundt in Heidelberg in 1858 – proposed what 112