Aristotle's De Anima! It's one of the West's foundational philosophical writings, investigating as it does that mode of being we call 'life'. Hence it is an ontological investigation. The pivotal concept is the _psychae_ (Latin: anima) which is the name for this mode of being itself: the principle of self-movement. Therefore the psyche is not a being, not an entity at all, and certainly not a material entity. De Anima therefore translates as 'On the Principle of Living Beings'. Life is that mode of being characterized by self-movement, that is, each living being, whether it be microbe, a plant or an animal, is 'enlivened' by its being able to move itself. 'Being able' designates a _dynamis_, a power within the living being itself: a _dynamis_ is itself an _archae kineseos_ or _archae metabolaes_ , i.e. a principle or starting-point for movement or change). A living being of any kind has a "principle of movement within itself", an _archae kineseos en autoi_.
Living being are only alive by virtue of partaking of the psyche, that is, in the principle of self-movement which covers reproduction, growth (through nourishment), decay (aging), maturing, locomotion (for animals). When a living being ceases to partake of this principle, it perishes, whereas mortals die, facing their death one way or the other (including refusing to face it).
The alternative? First and foremost, we human beings are all exposed to — i.e. ex-sist, stand out in — the openness of 3D-time, which is what I (following and interpreting Heidegger) propose as the alternative starting-point for thinking about what characterizes human being per se. As long as we mortals stand out in the three-dimensionality of time, we exist; we die when we can no no.longer do this, that is, when we depart the 3D-temporal openness.
This is an entirely different starting-point (_archae_) from Aristotle's in De Anima in which the human being is cast first of all as an animal that is then endowed in addition with _logos_ and _nous_, language and intellect. This legacy of starting from the animal as basis has led to the incorrigible materialism of today's scientific way of thinking and, in particular, most disastrously, in neuroscience, it has led to conceiving the mind itself (_nous_) as being somehow or other causally 'generated' by the material brain. But the principle of life itself, and the (temporal) principle of mind itself (also a kind of self-movement) cannot be caused by anything material. In particular, three-dimensional time is nothing material, and yet our mortal psyche and mind belong to it.
That doesn't stop modern bio-science from studying living beings with the ultimate aim of making life in the laboratory. Ontological impossibility cannot stop modern science because it doesn't think ontologically at all and therefore, on a deep level, despite all its complexity and sophistication, does not know what it is doing. It does not think simply enough about the simplest phenomena. Rather, it takes them for granted without further thought.
It seems to me that Western thinking failed to grasp the phenomenon of time from the beginning (that is, from Aristotle on); it missed the temporal phenomenon itself, reducing it to a mere derivative of movement/change and therefore as itself moving. Time accordingly 'passes' only as long as there is movement. If there's no movement, 'we' speak of 'timelessness', which means basically 'standstill'. Clock-time is the most vulgar kind of time, being as it is a number counted off movement, especially off the movement of the stars, sun and moon as observed from the Earth. Hence years, months, days, hours.
3D-time, by contrast. does not move; it is prior to movement altogether, enabling it: any movement/change can only occur in three-dimensional time by referring to present, past and future, to earlier and later, before and after. Otherwise, without 'standing out' into the 3D-temporal openness, we humans would know nothing at all of movement, we would not perceive it as such! We would also be totally ignorant of its negation: non-movement, non-change. The three-dimensional temporal openness is our primal gift.
Further reading: A Question of Time