This article challenges two dominant assumptions in the current ethical debate over the use of algorithmic Personalised Patient Preference Predictors (P4) in substitute judgement for incapacitated patients. First, I question the belief that the autonomy of a patient who no longer has decision-making capacity can be meaningfully respected through a P4-empowered substitute judgement. Second, I critique the assumption that respect for autonomy can be reduced to merely satisfying a patient’s individual treatment preferences. Both assumptions, I argue, are problematic: respect for autonomy cannot be equated with simply delivering the ‘right’ treatments, and expanding the normative scope of agency beyond first-person decisions creates issues for standard clinical decision-making. I suggest, instead, that the development of these algorithmic tools can be justified by achieving other moral goods, such as honouring a patient’s unique identity or reducing surrogate decision-makers’ burdens. This conclusion, I argue, should reshape the ethical debate around not just the future development and use of P4-like systems, but also on how substitute judgement is currently understood and justified in clinical medicine.