This paper examines fundamental problems underlying difficulties encou
ntered by pattern recognition algorithms, neural networks, and rule sy
stems. These problems are manifested as combinatorial complexity of al
gorithms, of their computational or training requirements. The paper r
elates particular types of complexity problems to the roles of a prior
i knowledge and adaptive learning. Paradigms based on adaptive learnin
g lead to the complexity of training procedures, while nonadaptive rul
e-based paradigms lead to complexity of rule systems. Model-based appr
oaches to combining adaptivity with a priori knowledge lead to computa
tional complexity. Arguments are presented for the Aristotelian logic
being culpable for the difficulty of combining adaptivity and a priori
ty. The potential role of the fuzzy logic in overcoming current diffic
ulties is discussed. Current mathematical difficulties are related to
philosophical debates of the past.