And as we all know, it came in a dream to a complete novice in chemistry with zero knowledge of any old ideas in chemistry: https://en.wikipedia.org/wiki/August_Kekul%C3%A9

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The empirical formula for benzene had been long known, but its highly unsaturated structure was a challenge to determine. Archibald Scott Couper in 1858 and Joseph Loschmidt in 1861 suggested possible structures that contained multiple double bonds or multiple rings, but the study of aromatic compounds was in its earliest years, and too little evidence was then available to help chemists decide on any particular structure.

More evidence was available by 1865, especially regarding the relationships of aromatic isomers.

[ Kekule claimed to have had the dream in 1865 ]

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The dream claim came from Kekule himself 25 years after his proposal that he had to modify 10 years after he proposed it.