To uneducated men it appears an indisputable fact of 'common sense' that the colour of a flower exists as perceived in the flower, apart from any relation to the percipient mind. A physiologist has gone further into the thicket of things, and finds that the way is not so simple as this. He regards the quality of colour as necessarily related to the faculty of visual perception; does not suppose that the colour exists _as such_ in the flower, but thinks of the something there as a certain order of vibrations which, when brought into relation with consciousness through the medium of certain nerves, gives rise to the perception experienced; and in order to account for the translation into visual feeling of an event so unlike that feeling as is the process taking place in the flower, physiologists have recourse to an elaborate theory, such as that of Helmholtz or Hering. In other words, physiologists here fully recognize that colour, or any other thing perceived, only exists _as perceived_ in virtue of a subjective element blending with an objective; the thing _as perceived_ is recognized as having no existence apart from its relation to a percipient mind. Now, although physiologists are at one with the philosophers thus far, it is to be feared that very frequently they are in the same position as the above-mentioned 'uneducated men,' when it becomes needful to press still further into the thicket. For after having distinguished the necessity of recognizing a mind-element in any possible theory of perception, they forthwith proceed to disregard this element when passing from the ground of perception to that of thought. Although the ideas of matter, motion, causation, and so on, are themselves as much the offspring of a thinking mind, with its environment, as the perception of colour is a conceiving of the percipient mind, with _its_ environment, these ideas are inconsistently supposed to stand for equivalent realities of the external world--to truly represent things that are virtually independent of any necessary relation to mind. Or, as the case has recently been well put by Principal Caird: