"No reflective construction would be possible without the initial choice which make us incline heart and mind for existence rather than non-existence." That is the Initial Postulate that Teilhard habitually ask to be granted at the beginning. He explains it in Letters from a Traveler:
Last night I had a long talk with the Doctor and another passanger on questions of moral philosophy. We finally had to admit that we differ in such fundamentals as: "Is it better to be or not?".I found this very in the line of Aquinas. First discuss being and non-being, then everything else follows.
I believe, in fact, that this is a fundamental option of all thought, a postulate that cannot be proved, but from which everything is deduced. Once it is admitted that being is better than its opposite, it is difficult to stop short of God; if it is not admitted, discussion ceases to be possible.
 L'energie Humaine (1937)
 Henri de Luback, Teilhard Explained, New York Paulist Press, 1968