[…] When…Abel is born in addition, Cain is eliminated.
This can be understood better if we examine the signification of their names.
Cain means ‘getting’ because he got everything for himself, Abel, on the other hand, did not, like his brother before him, refer everything to himself.
Devotedly and piously, he attributed everything to God, ascribing to his Creator everything that he had received from Him.
There are two schools of thought, therefore, totally in opposition one to the other, implied in the story of the two brothers.
One of these schools attributes to the mind itself the original creative source of all our thoughts, sensations, and emotions. In a word, it ascribes all our productions to man’s own mind.
The other school is that which recognizes God to be the Artificer and Creator of all things and submits everything to His guidance and direction.
Cain is a pattern for the first school and Abel of the second.
One living being gave birth to these two schools of thought. Hence, they are related as brothers because they come from one and the same womb.
At the same time, they are opposites and should be divided and separated, once they have been animated with the life of the spirit.
Those who are by nature contraries cannot abide for long in one and the same habitation.
Hence, Rebecca, when she gave birth to two individuals of dissimilar nature, the one good and the other evil, and when she felt them leap in her womb (Esau was the type of wickedness, Jacob the pattern of what is good), marveled at the reason for the discord which she perceived within her.
She appealed to God to make known the reason for her suffering and to grant a remedy. This was the response given to her prayer: ‘Two nations are in your womb; two peoples shall stem from your body’ (Gen. 25:23).
Interpreted spiritually, this can mean the same generation of good and evil, both of which emanate from the same source in the soul.
The former is likely to be the fruit of sound judgment whereby evil is repudiated and goodness is fostered and strengthened.
Prior to giving birth to what is good, that is to say, to giving complete reverence and deference owed to God Himself, the soul shows preference to its own creation.
When…the soul is generated with faith and trust in God, relief comes at the time of parturition.
Thus God, in applying the beneficial lesson of Abel to the soul of man, makes ineffective the impious lesson of Cain.
Ambrose of Milan (c. 337-397): Cain and Abel, book 1, chapter 1, 3-4, in St Ambrose: Hexameron, Paradise, and Cain and Abel, tr. John J. Savage, Catholic Univeristy of America Press, 1961, pp. 360-361.