In her 1917 philosophical essay, A Defence of Idealism, May Sinclair refers to dreams as “experiments with the soul that is to be”, and thus clearly challenges the notion of a transition between soul and psyche. As she discusses consciousness, individuation, the absolute, ultimate reality, sublimation and the libido in her philosophical essays and in her fiction, the word “soul” is central (it appears more than a hundred times) but never properly defined. After an analysis of Sinclair’s theorisation of “souls,” this chapter will explore how Sinclair represents souls in her fiction. Sinclairian souls always seem to refer to the irreducible individuality of her female characters and involve an active spiritual quest that relies on mystical visions, sensory keenness and rebellion against religion. But in Mary Olivier, Sinclair’s autobiographical Bildungsroman, Mary’s reappropriation of her soul induces an ambiguous “thrill of nothingness” that challenges Sinclair’s theoretical framework.