I. Introduction.- II. Reconstruction of the history of medieval and (post-) Cartesian theories of perception in terms of the negative heuristics of their respective research programs. Basic epistemological contrasts.- III. The formation of competing optical traditions in early and late antiquity.- (1) The various `optical' research traditions in early and late antiquity represent rival research programs into the theory of visual perception.- (2) The Aristotelian theory of vision.- (3) The Stoic-Galenic tradition.- (4) The geometrical tradition.- IV. The Identity Postulate at work in various research programs in the theory of vision during late antiquity and during the Arab and European Middle Ages.- (1) The Identity Postulate at work in the Stoic-Galenic theory of vision.- (2) The Identity Postulate at work in the geometrical tradition in the theory of vision.- (3) The Identity Postulate at work in Alhazen's theory of vision.- (4) The Identity Postulate reinforced by the Baconian-Alhazenian synthesis in optical theory. Internal explanations facilitated by the proposed rational reconstruction.- (5) The internal disintegration of the research program defined by the Identity Postulate during the 16th century.- V. The mathematization of physics and the mechanization of the world-picture gradually prepared in the development of medieval optics rather than in that of terrestrial or celestial mechanics.- VI. Mechanicism and the rise of an information theory of perception. A naturalistic reconstruction of (post-) Cartesian epistemology.- (1) Keplerian dioptrics, Cartesian mechanicism, and the rise of justificationist methodologies.- (2) Complete demonstration in science impossible. The need of conjectural theories affirmed.- (3) Ambivalence towards any alleged sources of `immediate' knowledge. Epistemology founded on an empirical theory of the senses and the mind.- (4) The rise of an information theory of perception. Internal tensions of the representationist research program.- (5) The representationist research program.- (5.1) Descartes against the identity theory of perception. The necessity of an information theory of perception.- (5.2) Two radical consequences of the new theory of perception.- (5.3) The negative heuristic of the Cartesian research program. Dualism of thought and sense. Descartes' information theory not a cognitive theory of perception.- (6) Malebranche and the Cartesian research program into optical epistemology.- (6.1) Ambiguities in Descartes' theory of sensory judgment. Lack of a genuine (cognitive) theory of information processing.- (6.2) Malebranche's theory of visual distance discrimination and of apparent magnitude.- (6.3) Regis contra Malebranche's information theory of perception. Corroborated empirical excess content of the Cartesian program according to Malebranche.- (6.4) Tensions between the positive and the negative heuristic of the Cartesian research program. The negative heuristic at work in Malebranche's theorizing.- (6.5) Rational reconstruction of Malebranche's occasionalism. Divine intervention and the computer analogy.- (7) Conclusion.- VII. Epistemological issues underlying the nineteenth century controversies in physiological optics. The Helmholtzian Program.- (1) The 18th century. Rationalist and empiricist developments. Cross-fertilizations of originally competing programs.- (2) The Helmholtzian research program into the theory of perception. The true logic of discovery revealed by rational reconstruction of the grand movement of intellectual history rather than by `faithful' intellectual biographies.- (3) The relevance of German Romanticism to the Helmholtzian program.- (4) Helmholtz's theory of subliminal cognitive activity.- (5) Helmholtz's research program contrasted with competing epistemological programs.- VIII. The interplay between philosophy and physiology in Helmholtz's view.- (1) Helmholtz's conception of philosophy in historical perspective.- (2) Muller's Principle of Specific Sense Energies.- (3) Helmholtz's theory of color vision.- (4) Helmholtz's theory of physiological acoustics.- (5) The philosophical significance of the Principle of Specific Sense Energies.- IX. Helmholtz's theory of the perception of space.- (1) Sensation and perception.- (2) The general idea of space and perceptual localization.- (3) The intuitionist theories of Muller and Hering.- (4) Helmholtz's empirical theory of perception.- (5) Methodological arguments in defense of the empirical theory of perception.- (6) The philosophical significance of the intuitionist-empiricist controversy.- (7) The general idea of space.- X. Helmholtz's theory of unconscious inferences.- (1) The need of an empirical non-introspective psychology.- (2) Helmholtz's theory not a mechanistic theory, but a truly cognitive theory of information processing.- (3) Helmholtz's theory of a continuum of cognitive functions beyond the edge of consciousness and beyond the grasp of verbal articulation.- (4) Helmholtz's theory dogmatically dismissed by the twentieth century ban on psychologism. Yet his cognitive theory superior as compared to traditional alternatives.- (5) The synthetic functions of subconscious mental operations according to 19th and 20th century theoretical developments. The problem of realism.- XI. The epistemological outcome of Helmholtz's naturalism. Hypothetical realism.- (1) Helmholtz's novel theory of causality in its relation to Kant, Reid and traditional empiricism.- (2) Lack of an adequate psychology. Weaknesses of Helmholtz's theory.- List of abbreviations.