It is not at all coincidental that Darwinian psychology has the same difficulty explaining the unity and integration of human reasoning as Darwinian biology has explaining the unity and integration of irreducibly complex functions. Practical and theoretical reasoning is often irreducibly complex. A given argument has several well-matched, interacting reasons, and the removal of any one of them makes the argument break down.
Angus J.L. Menuge, Agents Under Fire: Materialism and the Rationality of Science
logic, philosophy, psychology