Abstract:Base on the view of material monism, this paper explains how quantum mechanics describes the objectivity of the microscopic world. It points out that the challenge posed by the Copenhagen interpretation to the fundamental problem of philosophy is not essential, due to its adoption of the wave-packet collapse hypothesis which is non-unique to implement repeatable measurement. Therefore, the conclusion that matter and consciousness are inseparable is not rigorous either from scientific or from philosophical perspective. With regard to Karl Popper's philosophy of "three worlds", our quantum theory of measurement describes how the multiple observers probe into the micro system to obtain the objective knowledge about the microworld with objective quantum measurements, thus gives an ontological interpretation to the objective knowledge world (World 3) of Popper:the material world (World 1) interacts with the materialized carrier of spiritual perception world (World 2), forming the correlations or entanglements between the two worlds. These correspond to all mental perception including the subjective world. Here, the objective part, which can be defined by the objective quantum measurement, constitutes the objective knowledge world (World 3) of micro system. As the objective knowledge world emerges, the information flows from the material world to the subjective object. The direction of information flow defines the materialized carrier of spiritual perception, that is different from the usual material world.