Suppose that Q is a set of problems and S is a set of skills. A skill
function assigns to each problem q - i.e. to each element of Q - those
sets of skills which are minimally sufficient to solve q; a problem f
unction assigns to each set X of skills the set of problems which can
be solved with these skills (a knowledge state). We explore the natura
l properties of such functions and show that these concepts are basica
lly the same. Furthermore, we show that for every family K of subsets
of Q which includes the empty set and Q, there are a set S of (abstrac
t) skills and a problem function whose range is just K. We also give a
bound for the number of skills needed to generate a specific set of k
nowledge states, and discuss various ways to supply a set of knowledge
states with an underlying skill theory. Finally, a procedure is descr
ibed to determine a skill function using coverings in partial orders w
hich is applied to set A of the Coloured Progressive Matrices test (Ra
ven, 1965).