I am studying a particular physicist/philosopher and his views regarding reality outside of sentient beings. The term idealism is introduced and defined in his definition of terms, but I am a little confused over one of his specific variants of idealism that he defines, namely radical idealism. His definitions are as below in (1) and (2). The rest is entirely my interpretation. (1) Within temperate idealism the “thing in its self” notion is held to be meaningful, in spite of the fact that the said “thing-in-itself” is considered unknowable. (2) Within radical idealism, the “thing-in-its-self” notion is rejected. Temperate idealism I can understand and relate to – a rock is what we perceive, but the “source” of that rock, whilst “existing” in some scientifically indefinable form within mind independent reality cannot be said to have a one to one (or even close to that – we just cannot quantify the correspondence) correspondence to our perception of the rock. Radical idealism (as I understand it) does not acknowledge, even in principle, macroscopic objects as having any correspondence at all with a “source” that is external to our mind (in other words a “source” as “existing” within mind independent reality but of a form that we cannot say anything scientifically about). Rather, radical idealism would point to intersubjective agreement, our minds, and our consciousness as being the only ingredients required for a philosophical definition of our reality, there is no requirement whatsoever for any correspondence from our reality to an unknowable but nevertheless “existing” (in some indefinable form) “source” of our perceptions. Radical idealism expressed in this manner does not rest easily with me – to think of perceptions with no reference to “something” that gives (in part) rise to those perceptions seems to place knowledge as arising before existence and I find it hard to consider that intersubjective agreement can arise without reference to "something" external to our minds. Now I’m not sure whether I am taking this too far, is it actually the case that it is enough to say that radical idealism sees no point in taking on board a concept of mind independent reality if we can’t say anything scientific about it, but that it does not exclude the existence of “something”, or does it say (as I think to be the case) that it is a philosophical stance that does not in principle, consider there to be a need for any kind of “source” external to our reality that gives (in part) rise to our perceptions, in other words, our reality is a collection of minds connected together in terms of intersubjective agreement. This question is not just about labels, I want to be sure that there is a philosophical stance involving idealism that does reject absolutely the existence of any notion of “sources” relating to our perceptions as “existing” within mind independent reality, “sources” in this sense as not "existing" in any knowable form, but none the less, "existing" in a sense of giving rise to (in part) our perceptions, in other words, a “source” of intersubjective agreement. Any clarification of the above would be helpful to me – I did try googling the term radical idealism but there seemed little that was relevant to science.