This principle when expressed as an epistemological or metaphysical generalization, is called Phenomenalism.
"The Approach to Philosophy"
Ralph Barton Perry
And those who have cared more for this theory than for metaphysical speculation have attempted to stop at this point, and so to construe Phenomenalism as to make it self-sufficient on its own grounds.
"The Approach to Philosophy"
Ralph Barton Perry
In so far as subjectivism reduces reality to states of knowledge, such as perceptions or ideas, it is Phenomenalism.
"The Approach to Philosophy"
Ralph Barton Perry