Published online by Cambridge University Press: 05 April 2014
A favourite quotation by astronomers is a passage by the French philosopher, Auguste Comte (1798–1857). The nineteenth lesson of his Cours de Philosophie Positive appeared in 1835 and was one of several lessons dealing with the theory of knowledge in astronomy. With reference to the stars, he wrote:
We understand the possibility of determining their shapes, their distances, their sizes and their movements; whereas we would never know how to study by any means their chemical composition, or their mineralogical structure, and, even more so, the nature of any organized beings that might live on their surface. In a word, our positive knowledge with respect to the stars is necessarily limited solely to geometric and mechanical phenomena, without being able to encompass at all those other lines of physical, chemical, physiological and even sociological research which comprise the study of the accessible [i.e. terrestrial] beings by all our diverse methods of observation. 
A little later he continued: ‘I persist in the opinion that every notion of the true mean temperatures of the stars will necessarily always be concealed from us’ .
These passages may be amusing in the light of present knowledge, and it seems probable that Comte was ignorant of Fraunhofer's investigations from about 1814 to 1823 in which he described the absorption lines in solar and stellar spectra (see Sections 2.4 and 2.5).