From the Encyclical Letter Mater et Magistra of Pope John XXIII, May 15, 1961

( ... ) RESPECT FOR THE LAWS OF LIFE

We must solemnly proclaim that human life is transmitted by means of the family, the family founded on marriage, one and indissoluble, raised for Christians to the dignity of a Sacrament. The transmission of human life is entrusted by nature to a personal and conscious act, and, as such, subject to the all-wise laws of God: laws inviolable and immutable that are to be recognized and observed. Therefore, it is not permissible to use means and follow methods that can be licit for the transmission of plant or animal life.

Human life is sacred: from its very inception, the creative action of God is directly operative. By violating His laws, the Divine Majesty is offended, the individuals themselves and humanity degraded, and likewise the community itself of which they are members is enfeebled. ( ... )

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