When this view comes to be worked out as a philosophical theory, it usually undergoes some transformation. The fact of illusion, and the experimentally ascertained subjective differences between individual percipients,[46] or between different states of the same percipient, make it hard for the realist who wishes to be scientific to maintain that _all_ the perceived qualities of experienced things are equally independent of the experiencing subject. Reflection usually substitutes for the “naïve” realism of everyday life, a theory of “scientific” realism according to which the existence and some of the known properties of the experienced world are independent of the experiencing subject, while others are regarded as mere secondary effects arising from the action of an independent reality on the subject’s consciousness. With the further differences between the various types of scientific realism, according to the special properties which are held to belong to things independently of the percipient subject, we are not at present concerned.[47] It is, of course, clear that our general argument against the existence of any reality except as in an experience tells just as much against “naïve” realism and its more reflective outgrowth “scientific” realism as against Agnosticism. But the very plausibility and wide diffusion of realistic views of this type make it all the more necessary to reinforce our contention by showing what the truth Realism contains is, and just where it diverges from truth into fallacy. Nor is it specially difficult to do this. The important elements of truth contained in Realism seem to be in the main two. (i) It is certain that a thing may be real without being consciously present as a distinguishable aspect in my experience. Things do not begin to exist when I begin to be aware of them, or cease to exist when I cease to be aware of them. And again, the fact that I make mistakes and am subject to illusion shows that the qualities of things are not necessarily in reality what I take them to be. (2) Further, as is shown by the fact of my imperfect understanding of my own feelings and purposes, something may actually be an integral part of my own life as an experiencing subject without my clearly and consciously recognising it as such when I reflect on the contents of my experience.