‘What must our visual picture be like if it is to show us a transparent medium?’

Wittgenstein was motivated to write on colour, I have argued, because he realized he needed to explain why white surfaces cannot be transparent and this prompted him in turn to consider other questions about colour, some new, some he had previously discussed. In Part II he floats the idea that there is no transparent white since white is cloudy, and in Part III, III.135–153 he traces the impossibility to the ‘behindness’ and ‘see-through-ness’ of transparency. Now, at III.172, he takes up the problem a third time and discusses it again at considerable length, indeed continues to focus on it up to III.214 with subsidiary remarks at III.236–239 and III.252–253. There is no dividing line or other discernible break in the manuscript between the remarks published as III.171 and III.172 and no indication Wittgenstein is dissatisfied with the treatment of transparency and transparent white at III.135–153. He still takes transparency to be logically linked to ‘behindness’ and ‘see-through-ness’. But there is a noticeable shift of emphasis, and he is reasonably regarded as starting afresh after days, if not weeks, away from the topic. While he repeats some of what he says in earlier remarks, he mines a new vein and probes supplementary and complementary suggestions, all the while alert to the possibility that ‘transparent white’ may have application, i.e. it is grammatically coherent and at most empirically excluded.

The first sentence of III.172 reiterates the main thought about transparency conveyed at III.141–150. Wittgenstein states (without explanation or qualification) that the ‘impression of a coloured transparent medium’ is of something ‘behind the medium’. (I take ‘impression’ to refer –see II.1 –to the composite of shades of colour responsible for what is seen, not an image in the mind.) Wittgenstein is zeroing in on what he takes to be the logical point that a medium is transparent only in the event that objects lying behind it can be seen through it. Then in a second sentence he adds that he means a ‘thoroughly monochromatic visual image’, i.e. a visual image of a single hue, cannot be transparent.