There is no transparency without light and the association of light with knowledge exists from time immemorial. As the quintessential transparent solid, glass too is connected with knowledge. Galileo polished lenses to study the stars, as did Leeuwenhoek to show us the microscopic world. Spinoza himself was a lens maker, a fact always stressed in his biographies, though we do not know for sure how metaphysics behaves when it hits glass. Nonetheless, we should not forget that the transparent object does not always preserve the image that traverses it. Look no further than Newton’s prism, which decomposes white light into the colours of the rainbow, or the ‘diaphanous mantle of fantasy’ used by Eça to cover ‘truth’s harsh nakedness’.