This is the philosophical problem of the one and the many. According to the dual-aspect theory, both matter and mind are two different modalities or aspects of one underlying reality. I call this fundamental reality: Hyponoesis (Universal Mind). There is no independent substance of matter or mind besides Hyponoesis. Therefore, aspectuality just means a different manifestation of the same reality.
How can we explain the plurality of physical objects that are all aspects of Hyponoesis?
One answer, that is highly speculative but can be inferred from the nature of our own mind is the following: self-consciousness or self-reflection, that is unique to human beings, is a kind of division of a primary unity of our consciousness with nature. There is no inside and outside, no subject and object, but pure experience. An animal cannot reflect on its own behavior, it is not a subject as humans are. The animal therefore remains in the natural unity with itself and nature. But homo sapiens, at the time when self-consciousness in the form of self-reflection occurred, severed herself by this very act of self-reflection from nature. This self-referential act split our consciousness into subject and object, into mind and matter, into the inner world of experience and the outer world of physical objects.
Self-referentiality is the key to understanding how we perceive a multitude of objects, although there is potentially only oneness. In actuality, there is plurality, because our mind is capable of a contradistinction between itself as pure subject and its object as the matter of its reflection or thinking.
Similarly, Hyponoesis, in an act of self-reflection, produces the aspects of matter and mind. I understand these two modes (there are an infinite number of other modes (see Spinoza), but only mind and matter are available to our world) as two different levels of dynamistic or energetic structures (Plotinus' hypostases). These substructures can be compared to a different level of vibration of energy. Coarser vibratory levels are physical entities, whereas more refined levels correspond to mental entities. Both aspects are indicative of a multitude of phenomena. The interface between these two modes is the brain and consciousness as a direct output of neuro-physiological processes. As soon as mind interacts through the brain with the body, Individual Mind emerges (also called Personality in humans).
Exonoesis or the Individual Mind also splits up into a plurality of mental functions and faculties. Whereas all these faculties are local to Exonoesis (dependent on the operationality and capacity of the brain), Paranoesis (Transrational Thinking) is the interface with Hyponoesis (Universal Mind) and thus enables a reunion with Hyponoesis.
Matter, too, contains different aspects such as mass, electric charge, field, wave, particle, etc. These modes are produced through evolutionary processes (Big Bang, inflation-expansion, biological evolution, etc.) (see chart below).
(Some areas are clickable to get a definition of the term)
The modes of Exonoesis (Individual Mind) are produced through the primordial act of self-reflection, that creates language, culture, science, technology, etc.
All secondary modes are subject to change and transformation. They last only as long as a certain physical-mental entity or form exists. When either the physical part or the mental part ceases to function properly, the "actual entity" (s. Whitehead) is dissolved or transformed, that means, the secondary modes are canceled in the higher synthesis of the primary modes, e.g. will and intellect of a certain personality are canceled and dissolved in the primary mode of the Mind. The same happens with physical properties. The atoms or particles of the current form dissolve and create new forms or are transformed into energy or other material states.
The primary modes exist as long as Hyponoesis remains in the state of self-referentiality.