In response to this theory, much can be said that supports it. Firstly it appeals to common sense allowing for continuity through remembered experience. None of us are exactly as when we were children either physically or psychologically, yet we believe that we are the same individual as the child we once were. This belief is created by our memories.
A second argument in favour of psychological continuity is that it allows for physical change in a way that physical continuity cannot. Our bodies grow and regenerate and may contain different matter over time, our mental faculties provide continuity where matter cannot. For example, imagine that your mind was transferred into another body, would you not be the same person?
Furthermore, memory is unique in a way that the physical form is not. The physical form can be seen as sourced from many places, the ancestry of the individual for example, or the food that he has eaten. Memory on the other hand, whilst it can be influenced, allows for total subjectivity and thus uniqueness. This is crucial because the whole concept of identity relies upon uniqueness: identity means to uniquely discern one thing. If one identity can be confused with more than one person then a whole Shakespearean comedy of errors can follow.
Finally, Descartes' cogito (I think, therefore I am) can be used as an argument for the indubitably of mental states. As our access to our mental sates is immediate and direct then surely the mental faculties have primacy over the physical states. In observing our physical states we can be deceived and the sceptic can attack the argument, whereas we can be argued to have privileged access to our mental states. Thus, our continuity is not doubtful to us, as it surely must not be in order to maintain any sense of self.
Reide argued that points one and two showed that the old colonel did not remember being a boy and thus they cannot be the same individual. However, Butler pointed out that this would not function logically because if a equals b and b equals c then a must equal c.
Although this refutes Reide's central point, the issue of memory and what happens to one's identity if you forget memories remains. For example, suppose that I suffer amnesia after a blow to the head. I forget just a few hours of my life. Does this now make me less of an individual than my brother who was not injured? The issue of forgetting can be addressed by arguing, as Shoemaker did, that it is less memory but causal dependence that makes one psychologically connected with the earlier self.
However, it is possible to broaden the objection further. We can ask at what point does a person achieve an identity? How many memories have to be accumulated before one becomes an individual? Parents know that even the very young show signs of individuality and personality; this could be argued to be signs of psychological identity before the real formation of memory.
In relying on memory we must also be aware of the tricks it can play. We might remember our past with rose-tinted glasses, for example remembering every childhood summer as sunny and hot month after month. If memory provides us with a false or misleading record do we thus have a false identity? It would seem problematic to suggest so, but not a knockdown refutation of the theory.
Russell took this argument further by asking whether the theory would be compromised if we had all popped into existence five minutes ago with our memories implanted in us. It would seem from this consideration that memory alone cannot provide us with proof of the continuity of identity over time. If we assume that our memories have been implanted then we are betrayed from within in our attempts to introspect our identity.
A further significant objection to psychological continuity is the opposing physical continuity argument. This argues that the continuity of our identity is provided by the continuity of our bodies. This would immediately avoid Russell's objection (above) as our extension in time is only defined by the extension in time of our bodies. In addition this approach allows for psychological change without identity being compromised, thus allowing for amnesia or changes to personality.
More significantly, from the perspective of a material monist account of mind it could be argued that psychological continuity is the same as physical continuity and thus does not constitute a separate theory.
However, although physical continuity appears to solve the critical issue with psychological continuity (Russell's objection) it raises the issue mentioned above that it does not cater for physical regeneration. In this way physical continuity applies continuity rules as if to objects. Whilst a person has a physical presence at any given moment the matter that constitutes them is part of a dynamic system in a way that the matter of a static object is not. This applies to all living things and many non-conscious things as well (for example a puddle).
It seems there are issues with both models of continuity. The final argument against this standpoint will thus be to reject identity as Hume did with his bundle theory. In his introspective thought experiments, Hume failed to discover the 'I' that owned his experiences. Thus Hume believed that his 'bundle' of experiences associated only in that they were proximate, causally related and had a common thread formed his self. From this perspective there is no identity. Identity as it is referred to in language is something public and ever changing and does not persist through time. Your identity does not belong to you in this view, but is conferred upon you by people who need a method of differentiating you from other people they know.
There are obviously issues with the position of no identity. The fact we persist through time is central to our cognition. Without this how would intentional states be possible? The fact that other people persist through time in the same way is central to our communication (for example the use of names!). However, the way in which others place identity on you can be seen as separate to the internal experience of persistence and temporal awareness, something that Hume usefully contributes to the debate.
Therefore in conclusion to this assessment of the view that identity is psychological continuity we have seen that memory cannot provide us with a definite measure of determination through time. Hume's argument that we are just a bundle of experiences is convincing, but denies the existence of identity, which is itself tricky. It could be argued that the need for identity is social and linguistic. Physical and psychological properties of a person change constantly and consistent persistence of them through time is not possible or meaningful in the literal sense that these theories have examined.
