Which feudal class was NOT determined by birth serfs nobility clergy peasantry?

The class that was not determined by birth is the clergy. While many members of the clergy were born into noble or wealthy families, individuals from various backgrounds could enter the clergy through education and ordination. This allowed for some social mobility, contrasting with nobility and serfs, whose statuses were largely inherited. Peasantry, while also often hereditary, primarily included those who worked the land and did not hold the same spiritual or educational roles as the clergy.