thumb|Light spectrum, from Theory of Colours – Goethe observed that colour arises at the edges, and the spectrum occurs where these coloured edges overlap.
Theory of Colours () is a book by Johann Wolfgang von Goethe about the poet's views on the nature of colours and how they are perceived by humans. It was published in German in 1810 and in English in 1840. Unlike Newton, Goethe's concern was not so much with the analytic treatment of colour, as with the qualities of how phenomena are perceived. Philosophers have come to understand the distinction between the optical spectrum, as observed by Newton, and the phenomenon of human colour perception as presented by Goethe—a subject analyzed at length by Ludwig Wittgenstein in his comments on Goethe's theory in Remarks on Colour and in Jonathan Westphal's Commentary on this work (1991).
Historical background
thumb|250px|Reddish-yellow edges overlap blue-cyan edges to form green.
At Goethe's time, it was generally acknowledged that, as Isaac Newton had shown in his Opticks in 1704, colourless (white) light is split up into its component colours when directed through a prism.
right|thumb|250px|Castel's 1740 comparison of Newton's spectral colour description with his explanation in terms of the interaction of light and dark, which Goethe later developed into his Theory of Colours
This experience gave him the decisive impetus to develop his own theory of colour. and by 1793 Goethe had formulated his arguments against Newton in the essay "Über Newtons Hypothese der diversen Refrangibilität" ("On Newton's hypothesis of diverse refrangibility"). Yet, by 1794, Goethe had begun to increasingly note the importance of the physiological aspect of colours, "where it was even more difficult to distinguish between the objective and the subjective".
As Goethe notes in the historical section, Louis Bertrand Castel had already published a criticism of Newton's spectral description of prismatic colour in 1740 in which he observed that the sequence of colours split by a prism depended on the distance from the prism—and that Newton was looking at a special case.
"Whereas Newton observed the colour spectrum cast on a wall at a fixed distance away from the prism, Goethe observed the cast spectrum on a white card which was progressively moved away from the prism... As the card was moved away, the projected image elongated, gradually assuming an elliptical shape, and the coloured images became larger, finally merging at the centre to produce green. Moving the card farther led to the increase in the size of the image, until finally the spectrum described by Newton in the Opticks was produced... The image cast by the refracted beam was not fixed, but rather developed with increasing distance from the prism. Consequently, Goethe saw the particular distance chosen by Newton to prove the second proposition of the Opticks as capriciously imposed." (Alex Kentsis, Between Light and Eye)
Recently, experiments by physicist Matthias Rang have demonstrated Goethe's discovery of complementarity as a symmetric property of spectral phenomena. also a more recent reexamination of Newton's Experimentum Crucis has:
Current status
"Newton believed that with the help of his prism experiments, he could prove that sunlight was composed of variously coloured rays of light. Goethe showed that this step from observation to theory is more problematic than Newton wanted to admit. By insisting that the step to theory is not forced upon us by the phenomena, Goethe revealed our own free, creative contribution to theory construction. And Goethe's insight is surprisingly significant, because he correctly claimed that all of the results of Newton's prism experiments fit a theoretical alternative equally well.. a century before Duhem and Quine's famous arguments for Underdetermination."
"Goethe's critique of Newton was not an attack on reason or science, though it has often been portrayed that way.. The critique maintained that Newton had mistaken mathematical imagining as the pure evidence of the senses.. Goethe tried to define the scientific function of imagination: to interrelate phenomena once they have been meticulously produced, described, and organized... Newton had introduced dogma.. into color science by claiming that color could be reduced to a function of rays." (Dennis L. Sepper, 2009)
"Although he soon rejected Newton's differential refrangibility, Goethe always affirmed Newtonian mechanics. It was not an apriori poetic prejudice against mathematical analysis but rather performing the experiments that led him to reject the theory... Goethe soon concluded that in order to explain color one needs to know not just about light but also about eye function and relative differences in light across the visual field." (Sepper, 2009)
As a catalogue of observations, Goethe's experiments probe the complexities of human colour perception. Whereas Newton sought to develop a mathematical model for the behaviour of light, Goethe focused on exploring how colour is perceived in a wide array of conditions. Developments in understanding how the brain interprets colours, such as colour constancy and Edwin H. Land's retinex theory bear striking similarities to Goethe's theory.
A modern treatment of the book is given by Dennis L. Sepper in the book, Goethe contra Newton: Polemics and the Project for a New Science of Color (Cambridge University Press, 2003).