What visual characteristic makes architectural sketches such effective tools in creative processes or developing spatial ideas? In this article, I suggest that it is the visual characteristic of diaphaneity or the realm of the diaphanous – a nebula rasa – more generally. First, the history of the concept, as well as its relevance to creativity and design theory, is summarily discussed. This is followed by an exposition of how post-war design theory largely developed in a cognitivist direction, largely at the expense of lived experience. Invoking selected accounts from creativity theory and the so-called “phenomenological approach” in architectural thought as well as the work of sinologist and philosopher François Jullien, I shift the focus toward a new, agentive paradigm to re-orient current thinking on creativity and architectural design. Within this new conceptual framework, I develop the argument that the indistinct, suggestive, and allusive dimension of the diaphanous renders sketches effective. I argue that the nebula rasa drives the creative process and is instrumental in generating new ideas. The diaphanous regimen of visibility is inherently dynamic, continuously bringing new options in play and keeping drawings effective. In conclusion, I suggest how the concept of the diaphanous may benefit designing in a time of digital heuristics and increasing complexity. Moreover, I suggest that letting ideas rest may also be nurtured as a skill by engaging with the diaphanous.