Abstract: This paper begins by introducing the mineralogical concept of “Pseudomorphosis,” appropriated by Oswald Spengler in The Decline of the West. Drawing on cases from Spengler, Hans Jonas, and Hans Blumenberg, the paper analyzes the significance of pseudomorphosis for the study of the history of ideas, specifically in revealing the heterogeneous displacement between appearance and substance. The paper further proposes that this analytical method can be applied to architectural theory as illustrated in three cases: Étienne-Louis Boullée who ostensibly adhered to the Renaissance notion of “imitating nature” while actually implanting Cartesian mind-body dualism into architectural theory by emphasizing human “perception” and “sensation”; Gottfried Semper who continued Bötticher’s separation of core-form and art-form, yet shifted the philosophical basis from classical essentialism to Schopenhauerian pessimism, advocating for the use of “dress” to mask and reject reality; and finally, Louis Sullivan, whose romantic statement of “form follows function” was misread and distorted into mechanical functionalism, thereby betraying his original intent. These cases demonstrate that the concept of pseudomorphosis offers a specific analytical approach, facilitating a clearer discussion of complex relationships in architectural theory and unveiling the essential depth of architectural thought.