According to their opinion, all that we know, or perceive, or feel, or imagine about the world, is appearances or phenomena, but not reality itself. Appearances are 'things known as,' but not 'things as they are.' Thing-in-itself, or reality, lies behind appearances permanently beyond our ken. This is probably the most profound metaphysical pit into which philosophical minds have ever fallen in their way of speculation. Things appear, they would say, as we see them through our limited senses; but they must present entirely different aspects to those that differ from ours, just as the vibration of ether appears to us as colours, yet it presents quite different aspects to the colour-blind or to the purblind. The phenomena] universe is what appears to the human mind, and in case our mental constitution undergoes change, it would be completely otherwise.

This argument, however, is far from proving that the reality is unknowable, or that it lies hidden behind appearances or presentations. Take, for instance, a reality which appears as a ray of the sun. When it goes through a pane of glass it appears to be colourless, but it exhibits a beautiful spectrum when it passes through a prism. Therefore you assume that a reality appearing as the rays of the sun is neither colourless nor coloured in itself, since these appearances are wholly due to the difference that obtains between the pane of glass and the prism.

We contend, however, that the fact does not prove the existence of the reality named the sun's ray beyond or behind the white light, nor its existence beyond or behind the spectrum. It is evident that the reality exists in white light, and that it is known as the white light when it goes through a pane of glass; and that the same reality exists in the spectrum, and is known as the spectrum when it goes through the prism. The reality is known as the white light on the one hand, and as the spectrum on the other. It is not unknowable, but knowable.