Would you say that you are a body and brain, or the awareness that experiences them in the so called here now? Clearly, you cannot be both the physical entity and Awareness. Why? Because if you were both, you would have two distinct natures: one known, limited, conditioned, the other formless, unknown and unconditioned!
Is a "sentient being" the sum of various parts? It seems that the idea of "parts" and "wholes" can only appear within this instantaneously experiencing Mind!
Some insist that a human being is an activity or process in time. Would you say you are these words as you read them, the act of reading them, the realization that you have read them, feelings and judgments about what you have read -- or something else? It should be obvious that if you can hold all these parts of the process in time in Mind, objectifying them for analysis, you are not the parts or the process but the strangely receptive and still Mind which can never see the source of its own Seeing!
Listening to the rain fall, would you say that you are the sound of the rain, the sensory organs that transmit nerve impulses to the brain, the brain electrochemistry that registers these nerve impulses, the conscious realization of hearing the rain, or something else? Everything here can be broken down into "aggregates," distinct objects of thinking, except for the Mind that hears the rain and looks into the question of who or what hears it.
If you are the sound of rain, then when it stops you should be gone too. If you are just the immediate consciousness that hears the sound of rain, why should “you” last even a moment beyond it? If you do last beyond it, clearly you’re neither the sound nor the immediate awareness of the sound, but something else. In Zen this "something else" is not, however, something else. It is not a thing. It is not separate. Nor is it the same as "parts" and "objects." Nor is it a god.
As Lao-Tzu and Chuang-Tzu pointed out, such questions lead to an ultimate mysteriousness, a barrier before which thinking stops. Master Dogen says shinjin datsuraku -- "cast off [the conceptual illusion identifying primordial Awareness-Nature with] a mind-body." Then "cast off the casting off"!