Abstract AI safety, risk, and governance are becoming global agenda items, but the intensification of governance activity does not automatically clarify the object of risk. When different fields say that they seek to protect “humanity,” they implicitly point to preferences, lives, social order, or species continuity—concepts that are not mutually compatible. As a result, risk identification continually slides across dimensions. This paper is an exercise in conceptual engineering concerning the risk-bearing subject of AI-related existential risk to humanity. It also has significance for social epistemology, the philosophy of technology, and AI risk governance. This paper preserves the baseline of existing frameworks that understand “humanity” as Homo sapiens, but proposes a paradigm extension: the risk-bearing subject is always humanity as an embodied species; what this paper extends is not the risk-bearing subject, but the ways in which one and the same subject may suffer existential harm. The paper distinguishes two basic layers of possible harm—the biological-survival layer and the civilizational-conditions layer—and, within the latter, identifies the philosophical kernel, namely the category-calibration mechanism of foundational categories such as truth, evidence, responsibility, subjecthood, value, and meaning. AI may, through deprivation by substitution or mediated reconfiguration, weaken humans’ substantive participation in the functional systems of civilization, or cause foundational categories to lose stable calibration. Risk to the realization of subjecthood at the civilizational-conditions layer enters the scope of existential risk only when it acts upon foundational civilizational conditions, has diffusion capacity across domains and civilizational interfaces, may cause irreversible or difficult-to-reverse structural damage, and causes these harms to converge into overall, intergenerational damage to humanity’s capacity as a future-oriented subject. What this paper provides is not a complete taxonomy of risks or a governance scheme, but the ontological coordinate system needed for AI risk identification. The central point is not to add civilization, institutions, or the philosophical kernel as new risk-bearing subjects, but to show how the same human subject may suffer existential harm through damage to civilizational conditions. Files and repository This Version 3 record includes the English V3 PDF, the Chinese V3 PDF, and the complete source package. GitHub mirrors are available in the Trantor Labs Research repository.