The question of whether light is a particle or a wave dates back to at least Newton (who thought it was a particle) and Hyugens (who thought it was a wave). By

The question of whether light is a particle or a wave dates back to at least Newton (who thought it was a particle) and Hyugens (who thought it was a wave). By the end of the 19th century, before Einstein brought up the photoelectric effect, the consensus opinion was pretty firmly on the "wave" side of the dispute, and apparently Planck was not an outlier. See another comment: https://news.ycombinator.com/item?id=39349027 https://news.ycombinator.com/item?id=39349027