This chapter elaborates a theory of side thinking out of Cixous’s work. Side thinking has to do with previously unrecognised ways of thinking centre and margin, the explicitation of a logic of the side and side-effects, supplement and parergon, the effects of a thinking that operates on structures by a certain sideswiping or sidelining within. I develop this argument through a close reading of Cixous’s FirstDays of the Year (starting with its use of the eerie third-person formulation, thought the author, and its pervasive contention that ‘thinking is not what you think’), alongside Jacques Derrida’s H.C. for Life, That Is to Say… ‘Side thinking’ engages with a diverse array of topics, including amphibology, telepathy, literature and psychoanalysis, love and friendship, climate change, Brexit and nationalism. Extensive attention is given to the work of Samuel Beckett (in particular Endgame, Happy Days and Embers) and to Cixous’s book Zero’s Neighbour: Sam Beckett.