This paper studies two mechanistic explanations for human mortality by examiningreliability theory and vitality modelling through a unified actuarial perspective. While thetwo approaches arise from different ageing mechanisms, we show that both can naturallygenerate the Gompertz law under suitable assumptions and can be extended to producethe Makeham law and late-life mortality plateaus. Using Canadian mortality data, we in-vestigate the empirical behaviour of each approach and highlight the roles of heterogeneity,extrinsic risk, and stochastic randomness. Furthermore, we develop parallel definitions ofbiological age under both approaches and analyse how subjective survival beliefs emergefrom misspecified parameters. Our comparison of these two approaches provides actuar-ial insights into the natural foundations of Gompertz mortality and the interpretation ofageing, frailty and death.