This chapter focuses on Users (human or nonhuman) at the top layer up in The Stack and how they are interpolated by The Stack as comparable and interchangeable through various qualifications and quantifications of behaviors and impact. The User of this layer is not the universal persona that collapses design research into reductive and manipulative psychologism, a fixed term toward which design must orient its interfaces and artifacts, but as a model that is not given in advance and must be construed by interfaces and constructed for platforms. Its position at the top of The Stack, where driving agency is situated momentarily, is slippery, fragile, and always enmeshed in its own redefinition, an uncertainty that underwrites the formation of subjectivity in general, always a manifest image cobbled in relation to available technologies of self-reflection, from cave walls at Lascaux to Quantified Self Apps. <sup>8</sup> The Quantified
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User Layer