In this chapter, I discuss two influences shaping what I call 'the speculative method' in biography. As a historian who writes about past lives, I take as my starting point the idea of 'history's doubleness', which Ann Curthoys and John Docker coined in their book, Is History Fiction? This 'doubleness', they contend, comes from the fact that history is influenced by both scientific and artistic impulses, and it is these inherently entwined but often contested forces that make history not only analytical and imaginative but also inventive and 'self-transforming'. Here, I trace what nineteenth-century scientist William Whewell called 'the speculative nature' - firstly within science, and then 'art', by which I mean the narrative traditions underpinning both history and biography. I show that scientific speculation employs evaluative models that can be applied to speculative biography and that it is also useful to think of narrative as a laboratory in which biographers not only test their evidence and arguments but also experiment with suppositions as they inform their imaginations and construct plausible 'life worlds'.