This position, known as idealism or transcendental monism, is advocated by a number of scientists and philosophers (e.g. A. S. Eddington, J. Jeans, G. F. Stout, W. Harman). In conversation with Einstein, a famous Indian poet and philosopher Rabindranath Tagore defended, rather successfully, a (‘weak') version of idealism against realism. Idealism is not so absurd as it seems at first glance (especially if it is considered epistemologically rather than ontologically). In the end, what we know is the content of our mind (our perceptions are part of our mind not reality). And that content does not necessarily correspond to something ‘out there'. For example, the experience of solidity is only a mental product. As science confirms, a stone that appears solid is in fact mostly empty space. The usual refutation that goes something like ‘if you hit a table, the pain will be an obvious proof that it is real', does not, in fact, hold water. There is no reason why pain cannot be an established reaction to mental representations as much as material objects that have an independent existence. After all, we can experience pain in a dream as a result of hitting a dreamt table that is evidently not material.
A more serious problem that this position faces is how things unknown and unperceived by anybody can still exist. For example, somebody hides a treasure and subsequently dies. Nobody knows about it, but years later it is accidentally found. It makes sense to think that the treasure has existed all the time (although it was not in the mind of anybody). Bishop Berkeley, a philosopher often associated with this perspective, argues that reality is coherent and has continuity because it is in the mind of God - an omnipresent observer. Husserl also suggests in Cartesian Meditations that the transcendental ego would remain in existence even if the entire universe was destroyed. However, even if God or transcendental ego as a universal observer is accepted, another fundamental issue still remains. Whatever is called matter in our mental representations is distinct from non-matter (a dreamt table is, for all practical purposes, different from what is normally considered a real table). So, even if they are both subgroups of the mental, there is still a need to find out how these two subgroups relate to each other (e.g. what are hallucinations?). In other words, assuming that this perspective is correct would still leave the mind-body problem largely unresolved.
It can be concluded that neither materialist or idealist monism provide helpful arguments to overthrow the common sense assumption that both matter and mind exist and cannot be reduced to each other. This however, does not preclude the possibility that one causes the other, which is considered next.