This is an excerpt from the book Knights by Andrea Hopkins, page 30:
Together with the development of feudal society and the ethos that it fostered, there grew up another important and influential concept – that of the three estates, which together formed the natural order of society. This was an idea with a pedigree going back to classical times, and it was taken up again in the Middle Ages as the ideal social image. The three estates were the clergy, the workers and the warriors, or, in medieval terms, the Church, the peasantry and the knights. The Church’s function was to pray and to care for the spiritual well-being of the people, the knights’ to defend and protect the other two estates, and the peasants’ to work the land to provide food for everyone.
King Alfred the Great of England was one of the first medieval writers to promote this idea in his translation of the works of the much admired sixth-century philosopher Boethius, made about 894. Later ecclesiastical commentators, such as Adalbero of Laon and Gerard of Cambrai, also had much to say on the subject. Etienne de Fougeres, Bishop of Lisieux, is probably the first writer to identify the warrior class specifically as knights (“la chevalerie”) in the 1170s. This continued to be a powerful idea throughout the Middle Ages and still finds clear expression in the fourteenth-century allegorical poem Piers Plowman. In a famous passage Piers instructs others in how each must do his part in the right ordering of society:
“And all manner of men that live by meat and drink,Help those who win your food to work strongly.”
At this point a knight steps forward and offers to help Piers with his ploughing:
“By Christ,” said a knight then, “he tells us what is best,
But truly, I was never taught how to handle a team.
But teach me,” said the knight, “and by Christ I will try.”
”By St Paul,” said Perkyn, “You make such a fair offer,
That I will swink and sweat and sow for us both,And labour all my lifetime for your love,
In covenant that you keep Holy Church and myself
From wasters and from wicked men that destroy this world …”
Courteously the knight then uttered these words:
”By my power, Piers, I plight you my trothTo fulfil this agreement, even if I have to fight;
As long as I live I shall maintain you.”
The important point about this idea is that it links the knight’s purpose in life, his deeds and the credit attached to them, to the common weal of the whole of society. In conjunction with the feudal system, it broadened a knight’s responsibilities and extended his role beyond relationships governed by loyalty to kin and lord into a great interdependent structure of obligations extending, in theory, from peasant to king.