  People employ their knowledge to recognize things. This paper is concerned
with how to measure people's knowledge for recognition and how it changes. The
discussion is based on three assumptions. Firstly, we construct two evolution
process equations, of which one is for uncertainty and knowledge, and the other
for uncertainty and ignorance. Secondly, by solving the equations, formulas for
measuring the levels of knowledge and the levels of ignorance are obtained in
two particular cases. Thirdly, a new concept of knowledge entropy is
introduced. Its similarity with Boltzmann's entropy and its difference with
Shannon's Entropy are examined. Finally, it is pointed out that the obtained
formulas of knowledge and knowledge entropy reflect two fundamental principles:
(1) The knowledge level of a group is not necessarily a simple sum of the
individuals' knowledge levels; and (2) An individual's knowledge entropy never
increases if the individual's thirst for knowledge never decreases.
