Planner (often seen in publications as "PLANNER" although it is not an acronym) is a programming language designed by Carl Hewitt at MIT, and first published in 1969. First, subsets such as Micro-Planner and Pico-Planner were implemented, and then essentially the whole language was implemented as Popler by Julian Davies at the University of Edinburgh in the POP-2 programming language. Derivations such as QA4, Conniver, QLISP and Ether (see scientific community metaphor) were important tools in artificial intelligence research in the 1970s, which influenced commercial developments such as Knowledge Engineering Environment (KEE) and Automated Reasoning Tool (ART). Procedural approach versus logical approach The two major paradigms for constructing semantic software systems were procedural and logical. The procedural paradigm was epitomized by Lisp [McCarthy et al. 1962] which featured recursive procedures that operated on list structures. The logical paradigm was epitomized by uniform proof procedure resolution-based derivation (proof) finders [Robinson 1965]. According to the logical paradigm it was “cheating” to incorporate procedural knowledge [Green 1969]. Procedural embedding of knowledge Planner was invented for the purposes of the procedural embedding of knowledge [Hewitt 1971] and was a rejection of the resolution uniform proof procedure paradigm [Robinson 1965], which Converted everything to clausal form. Converting all information to clausal form is problematic because it hides the underlying structure of the information. Then used resolution to attempt to obtain a proof by contradiction by adding the clausal form of the negation of the theorem to be proved. Using only resolution as the rule of inference is problematical because it hides the underlying structure of proofs. Also, using proof by contradiction is problematical because the axiomatizations of all practical domains of knowledge are inconsistent in practice. Planner was a kind of hybrid between the procedural and logical paradigms because it combined programmability with logical reasoning. Planner featured a procedural interpretation of logical sentences where an implication of the form can be procedurally interpreted in the following ways using pattern-directed invocation: Forward chaining (antecedently): Backward chaining (consequently) In this respect, the development of Planner was influenced by natural deductive logical systems (especially the one by Frederic Fitch [1952]). Micro-planner implementation A subset called Micro-Planner was implemented by Gerry Sussman, Eugene Charniak and Terry Winograd [Sussman, Charniak, and Winograd 1971] and was used in Winograd's natural-language understanding program SHRDLU, Eugene Charniak's story understanding work, Thorne McCarty's work on legal reasoning, and some other projects. This generated a