This verse belongs to the Song of Enlightenment written by Yoka Genkaku. It is considered a fundamental text in Zen studies. One of the basic themes in the poem is the relationship between Buddha-nature and Self-nature. An also, I risk to say, between phenomenon and numinous. This is when Aristotle and Plato hold hands. Aristotle mostly inspired the scientific way of thinking while Plato gave some theoretical ground for religious philosophy to rely upon.
However, Yoka Genkaku in the song of Enlightenment does not concern itself with separation and categories as western philosophy usually does. The preferred approach of zen is to find communion among all beings, animate and inanimate, through our commonalities.
The ultimate meaning of the concept of “Emptiness” in Buddhism is that all beings are empty of identity, because we are all together one sole being. Before the 20th century, this was mostly a philosophical subject. With advent of Quantum physics Emptiness became a scientific certainty. Religion and science found a common ground. Belief and faith can be replaced by appreciation as it always being the attempt of zen practice. When a zen practitioner bows to the Buddha statue, the intent is actually to bow to the Buddha nature present in all individual selves.
Phenomena do not occur independent from each other. Individuality is just a conceptual tool to study and deal with the all-embracing, interconnected and interdependent reality. The reflection in the water, all the myriad beings, is just perceived as individuals due to the limitations of our human perception. Separation is an illusion. As every piece of knowledge, that realization brings us freedom but also bring us responsibility. It can set us free from loneliness and isolation. However, it also brings the inescapable necessity of seeing your neighbor as yourself. Therefore, you better love your neighbor as yourself as Jesus said.