“The natural philosopher must stick to the facts and try to find our their laws; and he has no means of deciding between these two kinds of speculation, because materialism, it should be remembered, is just as much a metaphysical speculation or hypothesis as idealism, and therefore has no right to decide about the matters of fact in natural philosophy except on a basis of fact. It is safer in my opinion to connect the phenomena of vision with other processes that are certainly present and actually effective, although they may require further explanation themselves, instead of trying to base these phenomena on perfectly unknown hypotheses as to the mechanisms of the nervous system and the properties of nervous tissue, which have been invented for the purpose and have no analogy of any sort. The only justification I can see for proceeding in this way would be after all attempts had failed to explain the phenomena by known facts. But, in my judgment this is not the case at all with the physiological explanation of visual perception. On the contrary, the more attentively I have studied these phenomena, the more have I been impressed by the uniformity and harmony everywhere of the interplay of the psychic processes, and the more consistent and coherent this whole region of phenomena has appeared to me.”

Hermann von Helmholtz