We have entered an era of intelligent system development in which fifth and sixth generation computing systems are expected to emulate and enhance natural intelligence. However, human knowledge processes are not well-understood and it is tempting to base the technology of intelligent systems on formal reasoning rather than human psychology. In particular, the attempt to base knowledge processing in fifth generation computing systems on logic programming suggests that we should examine the rationale behind standard logic. It was developed as a foundation for arithmetic and psychological considerations were ruled out as irrelevant. We should consider that there may be systems of logic more appropriate to knowledge processing. Logical and psychological system theories based on an axiom of unrestricted predication are plausible and attractive alternatives to standard logic. It is suggested that the design of logical inference processors should take such systems into account, and that investigation of the logical, psychological and systemic foundations of knowledge are fundamental to the development of intelligent systems.