How can we constitute the validity of this assumption?
The most reassuring argument for a materialistic world view seems to be the problem of the continuity of the world and the problem of the common sharing of the same world. This leads to the assumption, that matter exists independently of our consciousness, although it is conceded, that our mind modifies the positively given external impressions and sensations.
Let us tackle these two problems and see whether we could not explain them from a merely idealistic or even immaterialistic standpoint.
Whenever we go to sleep, we are almost a hundred percent sure to wake up again in the morning to the familiar world that we left the night before, and we can also be certain that the world continues to exist while our consciousness is faded during sleep. Do these certainties already allow us logically to assume an independently existing matter? It may be the easiest conclusion to common sense, but philosophers of all ages have always doubted this dubious induction.
How would a theory look like that excludes the independence of the world and have it determined solely by our thought? The only problem to be solved would be, how the world continues to exist during the sleep, although we do not actively keep on thinking. C.G. Jung introduced the concept of collective unconscious into psychology in order to explain inter-cultural phenomena and humankind's ubiquitous symbolism in mythology. Similarly, I propose a "collective thinking" or "collective thought" that sustains the continuous existence and order of the world. We are normally not aware of the background state of collective thought, since our world is embedded in the individual daylight thinking. Nevertheless, the world we think to perceive and think to be existent outside of ourselves is only a projection of collective thinking. However, I do not say, that only mind exists and matter is an illusion. Not at all, both mind and matter exist, and matter is as real as the mind, but matter is a product of the mind and not vice versa. With Mind I mean the Universal Mind (Hyponoesis). The illusion of matter must be assumed in the same way as the illusion of Exonoesis (Individual Mind). Ultimately, there's only Hyponoesis, one, undivided, non-dual source of infinite potentiality, of infinite thought.
The second argument against idealism is even stronger than the first one. If the world would be just a product of our mind (Exonoesis), so the argument goes, our view of the world would be totally subjective and every human being would perceive a world different from her neighbor's one. Communication would not be possible, because no two individuals could perceive the same thing, or even the same world. The world must be something given to our senses. The interpretation may be subjected to relativistic terms, but not the world-in-itself. Here again, assuming the collective thought as the determining power of the human mind, and based on the assumption, that all individual minds are actually one great Mind (Hyponoesis), we can easily discern the possibility, that we share in a common world by participating in collective thinking. Our perception is furthermore determined by the structure of our consciousness, as Kant's brilliant analysis of the mind shows.
There's a persuasive analogy: memory and the capacity to remember is not lost after waking up in the morning. Memory's contents is stored in the physical brain, but the capacity to retrieve the information from the memory and associate it in creative and imaginative ways is not a faculty located in the brain, but in the mind, which only uses the brain as an instrument of operation. The faculties of the mind are acquired and these capabilities, once learned, are not normally lost during an unconscious phase, such as sleep. Even in dreams are we able to produce a quite real world. Therefore, the acquired structures of thought (= our world view, the so-called external world) are sustained and are embedded in the continuous flux of change the things of our world undergo. The world does not exist independent of our thought. The world is not a product of our individual thinking (which is arbitrary and does not have the necessity of the physical world), but of mankind's collective thought, Hyponoesis.