“The duality of existence is completely contradictory to all available scientific training and human experience. Again, the ultimate proof of such affirmation is to experience one's self in this state of being.”

Source: The Book Thief

Fragments of Markham's notes
The Nemesis of Faith (1849)
Context: It is alike self-contradictory and contrary to experience, that a man of two goods should choose the lesser, knowing it at the time to be the lesser. Observe, I say, at the time of action. We are complex, and therefore, in our natural state, inconsistent, beings, and the opinion of this hour need not be the opinion of the next. It may be different before the temptation appear; it may return to be different after the temptation is passed; the nearness or distance of objects may alter their relative magnitude, or appetite or passion may obscure the reflecting power, and give a temporary impulsive force to a particular side of our nature. But, uniformly, given a particular condition of a man's nature, and given a number of possible courses, his action is as necessarily determined into the course best corresponding to that condition, as a bar of steel suspended between two magnets is determined towards the most powerful. It may go reluctantly, for it will still feel the attraction of the weaker magnet, but it will still obey the strongest, and must obey. What we call knowing a man's character, is knowing how he will act in such and such conditions. The better we know him the more surely we can prophesy. If we know him perfectly, we are certain.

Source: The Courage to Be (1952), p. 177

Letter Seven (14 May 1904)
Letters to a Young Poet (1934)
Variant: For one human being to love another human being: that is perhaps the most difficult task that has been given to us, the ultimate, the final problem and proof, the work for which all other work is merely preparation.
Source: The Selected Poetry of Rainer Maria Rilke
Context: People have (with the help of conventions) oriented all their solutions toward the easy and toward the easiest side of the easy; but it is clear that we must hold to what is difficult; everything alive holds to it, everything in Nature grows and defends itself in its own way and is characteristically and spontaneously itself, seeks at all costs to be so and against all opposition. We know little, but that we must hold to what is difficult is a certainty that will not forsake us; it is good to be solitary, for solitude is difficult; that something is difficult must be a reason the more for us to do it.
To love is good, too: love being difficult. For one human being to love another: that is perhaps the most difficult of all our tasks, the ultimate, the last test and proof, the work for which all other work is but preparation.

Book I, line 61. Quoted by Michel de Montaigne in Essays, Vol. III, Ch. 13 (tr. Charles Cotton).
Variant translation: Experience, after many trials, perfected the art, example showing the way.
Astronomica

The Limits of Evolution, and Other Essays, Illustrating the Metaphysical Theory of Personal Ideaalism (1905), Human Immortality: its Positive Argument

This is attributed to Pierre Teilhard de Chardin in The Joy of Kindness (1993), by Robert J. Furey, p. 138; but it is attributed to G. I. Gurdjieff in Beyond Prophecies and Predictions: Everyone's Guide To The Coming Changes (1993) by Moira Timms, p. 62; neither cite a source. It was widely popularized by Wayne Dyer, who often quotes it in his presentations, crediting it to Chardin, as does Stephen Covey in Living the 7 Habits : Stories of Courage and Inspiration (2000), p. 47. Such statements could be considered paraphrases of Hegel's dictum that matter is spirit fallen into a state of self-otherness. Or any number of thousands of similarly vague quotes by hundreds of predecessors.
Disputed
Variant: We are not human beings on a spiritual journey. We are spiritual beings on a human journey.
Variant: We are not human beings having a spiritual experience; we are spiritual beings having a human experience.