The argument I've chosen is in some sense the starting point of Finite and Eternal Being. "Whenever the human mind in its quest for truth has sought an indubitably certain point of departure," she says, "it always encountered the inescapable fact of its own being or existence" (FEB p. 35 - all citations are from the ICS translation). She cites in this connection Descartes, Augustine, and Husserl. The Sum, the "I am" is the most primordial knowledge available to us. It is not the first knowledge, in the sense of being temporally or logically first, but it is the most intimate and immediate knowledge one has, when one has it. This certitude of one's own existence is prior to reflection; in self-knowledge, she says, "the intellect relinquishes its natural attitude of being concerned with external objects in order to turn upon itself" (p. 37).
This being that I am, when brought into focus, has what Stein calls the dual aspect of being and not-being. I am "unable to endure this dual perspective," i.e., I experience this duality in my non-permanence or temporality; from this she concludes that "the being of which I am conscious as mine is inseparable from temporality" (p. 37). To be me is to be through time, to be in flux. But in this flux we as it were 'break apart'; our perspective is dual: we recognize both non-being and being in our change. This gives us the idea of pure being which, in itself, has no mixture of not-being, whether that of the no-longer or that of the not-yet. Pure being is eternal:
And thus eternal and temporal, immutable and mutable being (and also not-being) are ideas which the intellect encounters within itself; they are not borrowed from anything outside itself. This means that we have now found a legitimate point of departure for a philosophy base don natural reason and natural knowledge. (p. 37)
In recognizing being and not-being within ourselves, actuality and potentiality simultaneously begin to come into focus. The being manifested to ourselves in self-knowledge is manifested as actually present; but it contains in itself the potentiality of future actual being and presupposes the potentiality of past actual being: "My present being is simultaneously actual and potential being; and insofar as it is actual, it is the concrete realization of a possibility which antecedes my present actuality" (pp. 38-39). The ego, the I, lives, and is, and is actual, but not enduringly so. It is in a constant state of passing: it is a being thrown into existence [Dasein]. It has been placed into existence; it is received being.
Whence is it received? Either the ego receives its life from the worlds it experiences (external, internal, or both) or it owes its being directly to pure being itself. The two are not, of course, mutually exclusive. What is our relation to pure being? My own being is a being that involves not-being, as we have noted; but it also involves being "in touch with the fullness of being" (p. 55), as we also have noted. Once we grasp the idea of pure act or eternal being, this idea becomes the measure of our being - we approximate it in various ways from moment to moment. But, Stein asks, how do we learn to see eternal being as our source or cause? We get a feel for the nullity and transiency involved in our being from (among other things) existential dread:
Existential anxiety accompanies the unredeemed human being throughout life and in many disguises--as fear of this or that particular thing or being. In the last analysis, however, this anxiety or dread is the fear of being no more, and it is thus the experience of anxiety which "brings people face to face with nothingness." (p. 57)
(The quote is from Heidegger, Being and Time.) However, this angst (=dread or anxiety doesn't dominate our lives; we have what might be called a feeling of security, a trust in our own continuance. This is something to wonder at, because we are clearly transient creatures, indeed, always in flux and always a hair's breadth from death. Is this all due to an illusion or self-deception? Is it purely irrational? No, because this exposure to the possibility of nothingness is counterbalanced by our exposure to the certainty of being. In this we rest secure. But this security is not self-given; it is received, and thus, Stein notes, "In my own being, then, I encounter another kind of being that is not mine but that is the support and ground of my own unsupported and groundless being" (p. 58). There isn't much detail to this feeling of security; it needs to be clarified.
Stein identifies two ways in which we may become aware of eternal being as the source of our own being. The first is the way of faith, and the second is the way of discursive reasoning. In the way of faith, God reveals himself as I AM, as Creator, as Sustainer, as Life-giver. In the way of philosophy or discursive reasoning, she argues that my being, which is received being, and like all finite being, cannot have its ultimate origin and source in another received being. We must, as it were, trace ourselves back to necessary being. This converts the very vague feeling of security we feel in feeling ourselves anchored in being into something more robust. The way of discursive knowledge, however, fails to do much more than give us a clearer terminology, a tidied-up grasp of this very vague feeling; the way of faith, on the other hand, "reveals to us the God of personal nearness, the loving and merciful God, and therewith we are given a certitude which no natural knowledge can impart" (p. 60). But the way of faith is also a dark way; it is a condescension. She quotes Augustine's gloss on God's words in Exodus 3: "For this, I am who am--that pertains to me. But this, the God of Abraham and the God of Jacob, pertains to your understanding" (p. 60). The first is true, but in a sense incomprehensible; but the latter is adapted to our understanding. When it comes to knowing God, all ways are dark ways; the Light is so bright it blinds us.