Personal identity is a tricky subject in philosophy. It is tricky because when we analyse what it is to be the ‘same’, we find that very few things remain exactly the same as they persist throughout time. But clearly, we want to say that we are the same person that we were when we were a baby. In what sense of sameness then, do we mean when we say that we are the same person now as we were when we were a baby?
For two things to be the same, we might mean that they are qualitatively similar. When we say ‘those two footballs are the same’ we mean that those two footballs share the same qualities: they are both white, round objects, of about 15cm in diameter. It is important that we distinguish this sense of sameness from quantitative sameness, by which we generally mean ‘identical to’. ‘The person who writes Philosophy Tap’ and ‘the person who is writing this now’ are quantitatively the same, in that each predicate points to the same thing (me).
So one use of the word ‘same’ is in denoting things which are similar, and another is used to mean identical. The problem is with the use of the latter. As Leibniz pointed out, for two things to be quantitatively similar (identical), they must share the same properties. It might seem that the two footballs share the same properties, but even these differ in respect to their spatial properties (they each occupy a different part of space). So for two things to be identical, they must share every property, including spatio-temporal properties.
These definitions might seem clear-cut, but complications happen when we consider the concept of identity throughout time. If object A, on 15th March 2017, is said to be the same as object A on 1st March 2017, then it must share the same properties at both of these times. But all objects – and especially biological ones like human bodies – are constantly undergoing change in the form of physical and chemical alterations.
If it’s not having the same properties that define identity through time, then might it be something that cannot be captured qualitatively or quantitatively? What about consciousness – something that has so far eluded capture in terms of number or quality. Might it be consciousness that makes us the same, or, more precisely, the stream of consciousness particular to you or I, beginning from birth and continuing in to the future?
The problem with accounting for identity with consciousness is that consciousness also changes: rather than an uninterrupted stream beginning at birth and ending in death, consciousness is regularly interrupted by sleep, at which point it is no longer consciousness in our everyday sense of the word. More than this, the individual consciousness of you or I – the unique thoughts, feelings, memories, and introspection that each of us experience as our own, these are also constantly undergoing revision and change. Even if we do away with a definition of identity that requires the same properties, it seems that the degrees of change between the consciousness of three year old me and twenty-three year old me are too large to fit under one uniform identity.
Given these problems, how can I be the same person now as I was when I was a baby? One way of reducing – if not solving – the problem might be to argue that it is sufficient to count for identity that I am qualitatively similar to who I was when I was a baby (i.e. that I share some of the same qualities now as I did when I was a baby: I am born of the same parents, with the same hair and eye colour as when I was a baby). But rather than tackle the problem of identity, this answer seems to concede to the more pressing problem of identity – the problem of quantitative sameness.
If who we are is constantly changing – both our physical bodies and our consciousness, and, if what it is to be quantitatively the same is to hold the same qualities throughout time, then it seems that I cannot be the same person I was when I was a baby. If we are more generous in our conception of sameness – i.e. I have more or less the same thoughts, feelings, memories, and introspection as I did yesterday and the week before last, – then under such an account I am the same person as I was yesterday, or the week before last. But at the point where my conscious disposition changes: where my memories undergo revision (I forget what happened on my seventh birthday having previously known); or where I mature and my naivety changes to sound judgment, at this point it follows that I no longer have more or less the same feelings or memories as I did the day, week, or month before, and I must also not, therefore, be the same person as I was before.
For quantitative sameness to be salvaged, something platonic can be invoked: the form of person, or football, does not change throughout time. The types of these things: that football, you or I, are subject to change that mean we are never exactly the same when looked upon at two different times. Identity then, is not something linear, but a disparate collection of things – physical and other – extending through time.