Our sages, expounding on the ritual laws listed in our Parashah, explain that only something of value can become ritually unclean.
With this foundation we can understand why earthen vessels made of clay can only be rendered unclean from its contents and not from its exterior.
Earth has no intrinsic value, the value of an earthen vessel is that it can contain something within itself, therefore, contamination can only come from within the vessel, which is its point of value.
This law contains a moral lesson. G-d created man dust from the earth, and as with any other earthen item, man’s worth is within himself, and not external to him.
How different are the values of our society to those of our Torah. “How much is he worth?” is the criterion which our society employs in assessing the value of a human being. Money, fame and power reign supreme, and a person is valued for what he has, and not for what he is.
The Torah teaches us a different viewpoint, the primacy of the intrinsic worth of every human being is dependent on his character traits, and on what he achieves spiritually in life, rather than on his external acquisitions, whether these be fame or riches.