The Lord tells Aaron (and the priests): “You are to distinguish between the holy and the common, and between the unclean and the clean” (Lev 10:10 ESV).
What are the differences and relationships between “holy” and “common” and “clean” and “unclean”?
Gordon J. Wenham gives answers in his commentary on Leviticus in the NICNT series. Below are several quotations:
“Holy” is … the opposite of “common,” just as “clean” is the opposite of “unclean.”
Everyone and everything falls into one of two categories: the holy and the common.
Common things divide into two further subgroups: the clean and the unclean.
Cleanness is a state intermediate between holiness and uncleanness. Cleanness is the normal condition of most things and people.
Sanctification can elevate the clean into the holy, while pollution degrades the clean into the unclean.
Cleanness is the ground state; holiness and uncleanness are variations from the norm of cleanness.
The basic meaning of cleanness is purity.
Uncleanness is the converse of cleanness.
Permanent uncleanness cannot be altered and is not contagious, so no rites are prescribed to cure it.
Different degrees of uncleanness require different cleansing rituals.
Insistence on purification of the unclean is a corollary of the idea that Israel, the camp, and especially the tabernacle are holy.
Holiness characterizes God himself and all that belongs to him: “Be holy, for I am holy” (11:44-45; 19:2; 20:26).
Anyone or anything given to God becomes holy.
Uncleanness results from natural causes (e.g., disease) or human actions (e.g., sin), but holiness is not simply acquired by ritual action or moral behavior. Leviticus stresses that there are two aspects to sanctification, a divine act and human actions.
Usually, … the main emphasis of the book is on the human contribution to sanctification, what man has to do to make something holy.
… cleanness is the natural state of most creatures. Holiness is a state of grace to which men are called by God, and it is attained through obeying the law and carrying out rituals such as sacrifice. Uncleanness is a substandard condition to which men descend through bodily processes and sin. Every Israelite had a duty to seek release from uncleanness through washing and sacrifice, because uncleanness was quite incompatible with holiness of the covenant people.