Niels Bohr regarded the “complementarity principle” as the most fundamental principle of Quantum Mechanics, He introduced the concept of “complementarity” in his Como Lecture in 1927. The formal presentation  was given in his book “Atomic theory and the description of nature” (Cambridge University Press) in 1934. In his 1958 paper “Quantum physics and Philosophy: Causality and Complementarity”  Bohr explains the “complementarity principle” as follows.
“Within the scope of classical physics, all characteristic properties of a given object can in principle be ascertained by a single experimental arrangement, although in practice various arrangements are often convenient for the study of different aspects of the phenomena. In fact, data obtained in such a way simply supplement each other and can be combined into a consistent picture of the behaviour of the object under investigation. In quantum mechanics, however, evidence about atomic objects obtained by different experimental arrangements exhibits a novel kind of complementary relationship. Indeed, it must be recognized that such evidence which appears contradictory when combination into a single picture is attempted, exhaust all conceivable knowledge about the object. Far from restricting our efforts to put questions to nature in the form of experiments, the notion of complementarity simply characterizes the answers we can receive by such inquiry, whenever the interaction between the measuring instruments and the objects form an integral part of the phenomena.”
Specifically, what “novel kind of complementary relationship” is Bohr talking about? One often cited example is the complementarity of “particle” and “wave” views. Some experiments detect the wave nature of the elementary particles and other experiments detect the particle nature. Bohr is saying that both views are valid and complementary.
Neils Bohr is the main architect of the Copenhagen interpretation of Quantum Mechanics. The “complementarity principle” is part of the Copenhagen interpretation. Bohr’s “complementarity principle” has philosophical implications.
According to some physicists, one of the philosophical implications of the “complementarity principle” is the view that elementary particles do not have intrinsic properties independent of the measuring device. I suppose the supporters of this view use the term “measuring device” in the most general sense. For example, any external (classical) field would be a measuring device. I suggest the term “measuring base” for this. If I go along with this line of thinking I should interpret the external field created by the attractive electric force between the protons in the nucleus and the orbiting electron as a measuring device. According to this view the measured properties of electrons in an atom such as charge, spin and mass are manifested by the local environment of the atom.
The “complementarity principle” is a very useful principle but I don’t think that it implies the view that elementary particles do not have intrinsic properties independent of the measuring device or measuring base. The theoretical physicists who constructed the Standard Model (SM) of particle physics, and the experimental physicists who built the Large Hadron Collider (LHC) and its many detectors would not agree with this view either. The reason for their rejection would be that charge, spin and mass of elementary particles are measurable in a very consistent way across many different local environments (measuring bases) very different from the atomic environment.
All electrons are identical because they all exhibit the exact same charge, spin and mass. It is true that one experimental setup may extract the wavelike behavior of electrons and another experimental setup may extract the particle behavior but other properties known as charge, spin and mass are still needed to explain the wavelike behavior as well as the particle behavior. In the light of the latest experimental findings it would not be accurate to say that charge, spin and mass do not exist independent of the “measuring device.”
There are intrinsic properties of elementary particles. The term “intrinsic property” does not mean that an elementary particle is a “thing-in-itself.” Elementary particles are temporary formations of a primordial field. That primordial field itself may consist of subtler units called cittanu or maybe the primordial field is made of microvita, who knows! Physicists may come up with different concepts such as strings, filaments, membranes, branes, etc. Ultimately, all these formations are internal to the Cosmic Mind.
There are many subtleties regarding the role of observers in physical phenomena but don’t forget that at the most fundamental level the physical universe is observed and witnessed (substantiated and given life to) and controlled by the Cosmic Soul through the Cosmic Mind. The physical universe as well as the mental and the spiritual universes are embedded in the Cosmic Mind and all exist because of the Cosmic Soul. Many futile discussions take place because everyone seems to forget this fundamental fact. Well… some “know-it-all” philosophers and scientists do not actually forget, on the contrary, they do everything in their power to make us forget about this fundamental fact.
The ultimate measuring base is the Cosmic Mind – the Mind of God.
All elementary particles including the theorized ones that represent subtler formations within the physical realm are “relative truth” which is objective by definition. The “objective” is not necessarily physical. Misunderstanding of this fact is the source of endless discussions.
Please take a look at my “Relative Truth is Objective” article for more on this.
 Neils Bohr, “Atomic theory and the description of nature.” Cambridge University Press (1934)
 Neils Bohr, “Essays 1958-1962 on Atomic Physics and Human Knowledge.” Wiley (1963)