Through the journal entries of Theo in The Children of Men we are provided with an in-universe perspective of the change in humanities spirit in P.D. James grim vision of the future. 

Through Theo’s eyes passion has died in the face of modernity and disillusionment.  Presaging the loss of man’s ability to reproduce he witnessed the slow death of the great ocean that is humanities ability to feel.  But feel what?  The sensations of the flesh?   No, but Theo saw the loss of desire even before the Omega generation was conceived.  The drives of the body are co-morbid with the state of the mind.  So the root of the moratorium of humanities spirit must be elsewhere. 
The answer I believe is in the loss of not just the ability to love, but the ability to feel with passion.  Humanity did not lose just one hue of the spectrum of passions.  Humanity lost all of them.  There is no great war.  There is no longer place for the youth, the Omega’s, to prove their strength.  Passionate love and hate, fear and courage, these things are gone.   In their place what is left?  Nothing.  Moratorium of the spirit overtook humanity even before Omega in Theo's eyes.

P.D. James creates a vision of the slow death of humanity in her novel The Children of Men (1992).  James creates a bleak picture of the future beginning in 1994 when mankind loses its ability to reproduce, by the following year the last generation of man is born, the Omegas. The story of the slow death of the spirit, ageing of the body, and gradual extinction of mankind is presented from multiple perspectives, third and first person.  The description of the dying spirit of civilization comes from journal entries contemporary to the setting in 2021.   These entries are written by Theo, an Oxford history professor and former advisor to Xan, the Warden of England and Theo's cousin.  The entries provide an in-universe perspective to the slow extinction and disillusionment of humanity. From Theo’s perspective the entire world is looking to the past for comfort rather than face their reality.  His clear rejection of religion shows a faith in science, his preoccupation with 19^th century history shows a possible influence of Nietzsche.  Indeed the philosopher’s epic declaration, “God is dead”, seems to echo throughout the novel.  It would be suitable for another declaration, “Humanity is dead.” 

The possibility of redemption for humanity and the rebirth of the race seems a far off possibility as homes are abandoned and mass suicides are accepted. Without the miracle of birth, humanity has grown to accept death.   And after years of watching the slow extinction of a species the question emerges, what would it take to arouse humanity from its lethargy and revive its dying spirit?