Two important concepts within invention were topoi and stasis. Other rhetorical cultures seem to have additional means of locating "available means". Historian of Celtic poetics Robert Graves credited analepsis as a method of inventing his historical arguments in "The White Goddess", and the Mazatec medicine woman Maria Sabina credited the hallucinogenic psilocybe mushroom with the flow of her discourse. The philosopher Jacques Derrida described "inventio" as the "invention of the other." Janice Lauer proposes that invention should be: (1) applicable to a wide variety of writing situations so that they will transcend a particular topic and can be internalized by the student; (2) flexible in direction allowing a thinker to return to a previous step or skip to an inviting one as the evolving idea suggests; and (3) highly generative by involving the writer in various operations—such as visualizing, classifying, defining, rearranging, and dividing—that are known to stimulate insights. In classical rhetoric, arguments are obtained from various sources of information, or topoi (Greek 'places'; i.e. "places to find something"), also called by the Latin name "loci" ("cf". Literary topoi). Topoi are categories that help delineate the relationships among ideas; Aristotle divided these into "common" and "special" groups. In the common group could be found such categories as laws, witnesses, contracts, oaths, comparisons of similarity, difference, or degree, definitions of things, division of things (whole or parts, for instance), cause and effect, and other items that could be analyzed, researched, or documented. Modern writers and students use these topics, as well, when discovering arguments, although today more emphasis is placed on scientific facts, statistics, and other "hard" evidence. Classical rhetoricians saw many areas of inquiry that today's writer might view as being purely in the province of "logic", developing syllogisms, finding contradictions, as being of equal or greater importance. Barbara Warnick has compared the 28 topics of Aristotle's "Rhetoric" and topical schemes of Chaïm Perelman and Lucie Olbrechts-Tyteca's "The New Rhetoric" to illustrate major differences of rhetoric throughout these time periods. For example, two of Aristotle's topics "Opponent's Utterance" and "Response to Slander" were more relevant to ancient debates in the practice of Athenian law, in which every citizen was his or her own lawyer.