The 1970s saw some reaction against the rationality of design methods, notably from two of its pioneers, Christopher Alexander and J. Christopher Jones.[17] Fundamental issues were also raised by Rittel, who characterised design and planning problems as wicked problems, un-amenable to the techniques of science and engineering, which deal with "tame" problems.[18] The criticisms turned some in the movement away from rationalised approaches to design problem solving and towards "argumentative", participatory processes in which designers worked in partnership with the problem stakeholders (clients, customers, users, the community). This led to participatory design, user centered design and the role of design thinking as a creative process in problem solving and innovation.