This essay examines how radical evangelicals employed psychological concepts such as sanity, temperament, and especially the subconscious as they struggled to understand and respond to the rapidly expanding pentecostal movement within their midst. By tracing the growing tensions over ecstatic spiritual experiences that emerged among Holiness and Higher Life believers during the 1880s and 90s, this article demonstrates that differing assumptions about the importance of consciousness for the religious life presaged reactions to the pentecostal revivals of the early twentieth century. Although their proclivity for rational judgment predisposed Higher Life evangelicals to question the sanity of involuntary phenomena like speaking in tongues, some prominent leaders within this community appealed to “mental science” in an effort to revise conventional understandings of the spiritual self and its capacities. For participants in the Christian and Missionary Alliance – an organization in which disputes over the propriety of pentecostalism were particularly contentious – notions of temperament and the subconscious articulated in the works of “new psychologists” like William James offered resources for reassessing Higher Life views of authentic spirituality in light of pentecostal revivalism. By analyzing how a particular faction within the radical evangelical movement made use of psychological theories to contend with the challenge of the revivals at Azusa and elsewhere, this essay exposes some of the social divisions that exacerbated debates over the validity of pentecostal religious experiences. Exploring the complicated interactions and creative tensions that arose as Higher Life evangelicals appropriated constructs such as the subconscious in the wake of Azusa Street also shows that this influential contingent of conservative Protestants engaged with aspects of the field of psychology in dynamic and inventive ways that involved both selective borrowing and critical resistance. While there is truth in the common observation that radical evangelicals were deeply suspicious of the “new science of Psychology,” this article uncovers a more complex history that expands our understanding of the interplay among scientific discourse, the varieties of evangelical spiritual experience, and the emergence of pentecostalism in the early twentieth century.