Tuesday, April 02, 2024

Parfit on brain states and personal identity

Derek Parfit (1942-2017) was a British philosopher who specialized in personal identity, rationality, and ethics. A professor at Oxford he is best known for his paring back of personal identity in the Humean tradition, denying any core subject, self or soul.

He was analytic, believing in the power of reason but also thought experiments to expose weaknesses and strengths around personal identity and ethical issues. His work continues to be hugely influential in the fields of philosophy and ethics.

Personal Identity

His most famous work is Reasons and Persons (1984) where he challenges conventional views of personal identity and discusses the implications of such views for ethics and rationality.

The single, continuous self is an illusion, what matters is psychological continuity and connectedness not personal identity. Evolved for survival, we rely on memory, personality, and intention, not any sense of the moral self. He holds that that personal identity comes down to particular facts about brain states, psychological continuity, and bodily continuity. There is no need for any additional ‘subject’ as self and that speculation beyond these facts is pointless and fictional.


This he believes has huge consequences in terms of ethics and in On What Matters () where a complex mix of reasoned ethics lifts us out of mere feelings and emotions. This view of personal identity breaks down the sovereign self into a more social and shared view of ethics.

He has a Triple Theory, where Kant, consequentialism, and contractualism come together to support a common set of ethical standards. His Kantian Ethics saw certain rule-based moral laws that we must follow based on the Kantian idea of universalizability. Then there is his rule consequentialism, where we agree to follow rules that are in our best interest in terms of consequences. This allows for the consideration of the consequences of actions, but within the constraints of rule-following. Finally, there is contractualism the idea that something is right or wrong for informed, unforced general agreement.

Critique and consequences

When personal identity is not seen as singular and necessary for survival it raises challenges around personal identity, responsibility and institutions that rely on that concept. It also raises questions about the status of psychological continuity. 

One can see how this idea, when applied to AI leads to us confusing AI by reading subjects, or selves into the machine, ghosts in the machine. When that goes, as it does in Large Language Models, then we have a more diffuse entity with less individualistic outputs. Without the connectedness and continuity of being a person, can AI have a sense of being in the world?


Parfit’s philosophical work has had a substantial impact on fields like ethics, philosophy of mind. His work raises questions in AI around what being human is and the consequences of a shallower, brain-process orientated sense of identity. He has been influential on theorists who see the brain He did not propose a computational theory of the brain in the sense that cognitive scientists or computational neuroscientists might buy laid the ground for later theorists to layer this on to his ideas.


Parfit, D. (2011). On what matters (Vols. 1 & 2). Oxford: Oxford University Press.

Parfit, D. (1997). Equality and priority. The Lindley Lecture. Lawrence: University of Kansas, Department of Philosophy.

Parfit, D. (1984). Reasons and persons. Oxford: Clarendon Press.


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