irreducible complexity

Irreducible complexity (IC) is the argument that certain biological systems with multiple interacting parts would not function if one of the parts were removed, so supposedly could not have evolved by successive small modifications from earlier less complex systems through natural selection, which would need all intermediate precursor systems to have been fully functional. This negative argument is then complemented by the claim that the only alternative explanation is a "purposeful arrangement of parts" inferring design by an intelligent agent. Irreducible complexity has become central to the creationist concept of intelligent design (ID), but the concept of irreducible complexity has been rejected by the scientific community at large, which regards intelligent design as pseudoscience. Irreducible complexity and specified complexity are the two main arguments used by intelligent-design proponents to support their version of the theological argument from design.

The central concept that complex biological systems which require all their parts to function could not evolve by the incremental changes of natural selection, and so must have been produced by an intelligence, was already featured in creation science. The 1989 school textbook Of Pandas and People introduced the alternative terminology of intelligent design, a revised section in the 1993 edition of the textbook argued that a blood-clotting system demonstrated this concept.