Screwtape articulates his theory on human existence as “undulation—the repeated return to a level from which they repeatedly fall back, a series of troughs and peaks” (37). Because humans are partly physical beings, and the material world is always in flux, people naturally find it difficult to remain focused on any single thing. Screwtape therefore warns that the patient’s commitment to Christianity may not truly be waning, as Wormwood believes. Indeed, humans will often come closer to God during “troughs” of emotional investment in faith.

Screwtape continues to elaborate on “troughs” in human life. He suggests that these periods offer opportunities for all sensual temptations, “particularly those of sex” (43). However, Wormwood must be careful not to confuse pleasure itself for sin, as any sort of bodily enjoyment ultimately comes from God and is not, in its “healthy” state, sinful. The best way to exploit the patient’s “trough” is therefore to convince him that it will last forever and perhaps that it was foolish of him to have ever felt strongly about his religious convictions.