Existential simplicity of architecture can establish harmony between the field of human life environment (architecture), the field of human inner and soul, and the field of existence and reality (ontological field). It is also important in the context of transcendental wisdom especially while the rule of " "Simple al-Haqiqah" is ongoing. By using the method of library documents, and a combination of logical reasoning methodology besides qualitative content analysis, the author initially reviews the existing. After that, by examining the foundations of transcendental wisdom, he takes a step towards Sadra's epistemology and tries to match the obtained components with the corresponding components in the architectural design process models, and then explains how the foundations obtained from transcendental wisdom affect the design processes. The result is that the simplicity of the soul in relation to its internal powers is surrounded by the simplicity of the necessary being in relation to the entire system of existence, and since architecture is also a function of the powers of the soul, including the power of imagination; Therefore, surrounded by the simplicity of the self is raised and this important occurs in the context of Mulla Sadra's basit al-haqiqa rule - as the most fundamental principle in the realm of ontology and epistemology that accepts plurality with the appearance of unity in multitudes - and its consequence is Sadra's design process.