Do what thou wilt shall be the whole of the law is Aleister Crowley's Thelemic maxim, and it in turn echoes Francois Rabelais's fictional Abbey of Theleme, whose sole rule for its inhabitants was do what thou wilt. Gerald Gardner had contact with Crowley in the 1940s, and Wiccan tradition inherited the same structural idea, a single permissive rule, but qualified so that its permission does not become licence to harm others. The rider, an it harm none, is what turns Crowley's maxim into something recognisably Wiccan rather than Thelemic, and it is this rider, not the older do what thou wilt formula, that is distinctive to Wicca.
In practice the Rede functions less as a rule with a court behind it and more as a discipline of self questioning, since Wiccans are expected to weigh out beforehand who and what an action might harm, including the practitioner, rather than to treat the words as a permission slip. The longer, twenty six line poem that closes on the same couplet has done more to popularise the phrase than any Gardnerian source, since it circulated widely through 1970s and 1980s Pagan periodicals regardless of lineage. Whatever its authorship, disputed as it is, its ethic has become close to a universal reference point across Wiccan and much eclectic neopagan practice, cited far more often than any doctrinal statement about deity or afterlife.