In his book The Emperor’s New Mind, which was first published in 1989, Roger Penrose discussed the paradox that light can, depending on the way that an experiment is conducted, exhibit particle-like or wave-like behaviour:
Let us consider the “archetypical” quantum-mechanical experiment, according to which a beam of electrons, or light, or some other species of “particle-wave”, is fired through a pair of narrow slits to a screen behind […]. To be specific, let us take light, and refer to the light quanta as “photons”, in accordance with the usual terminology. The most evident manifestation of the light as particles (i.e. as photons) occurs at the screen. The light arrives there in discrete localised units of energy […] Never is the energy of just “half” a photon (or any other fraction) received. Light reception is an all-or-nothing phenomenon in photon units. Only whole numbers of photons are ever seen.
However, a wavelike behaviour seems to arise as the photons pass through the slits. Suppose, first, that only one slit is open (the other being blocked off). After passing through, the light will spread out — by the phenomenon called diffraction, a feature of wave propagation. […]
However, the key problem for the particle picture occurs when we open the other slit! […]
There is nothing puzzling about an ordinary macroscopic classical wave travelling through two slits at once in this way. […] But here things are very different: each individual photon behaves like a wave entirely on its own! […]
Does the photon actually split in two and travel partly though one slit and partly through the other? Most physicists would object to this way of phrasing things.
If we postulate that, in the realm of metaphysics, there may exist a principle of duality similar to the wave-particle duality, it may perhaps help elucidate some longstanding problems:
The doctrine, expressed in the Nicene Creed and the Athanasian Creed, that Jesus is both fully man and fully God.
The apparently irreconcilable contradiction between predestination and free will.
The belief that the Old and New Testaments are divinely inspired, although any passage of OT and NT may clearly display characteristic traces of its human author.
The doctrine of transubstantiation. Some but not all Christians adhere to this doctrine, which maintains that the communion bread and wine remain bread and wine, and yet become the body and blood of Jesus.