Stumpf began his "Tonpsychologie" (Tone psychology) in 1875, perhaps his greatest contribution to psychology. He distinguished between phenomena and mental functions, suggesting that phenomena such as tones, colours, and images are either sensory or imaginary. Stumpf termed the study of such phenomena as "phenomenology". He did a wide range of studies of the sounds of different instruments, melody, tonal fusion, and the consonance and dissonance of tones. 