Project 1:

Can Thought Experiments Solve Problems of Personal Identity?

Thought experiments are mental test scenarios that purport to deliver scientifically acceptable results in the absence of actual physical execution. Scientists use imaginary situations as a method to test hypotheses, to expose contradictions, or to explore the scope of concepts. Hypothetical reasoning is employed in a variety of disciplines, including in physics and economics, and it has a particularly long and important tradition in philosophical discourse, which began as early as in pre-Socratic times. Also engaging in a meta-discourse on the thought-experimental technique, however, is a comparatively recent phenomenon.


The thought-experimental method has had many prominent advocates, including such major figures like Descartes and Leibniz. Especially in debates about personal identity, philosophers have been relying heavily on thought experimentation, and the intuitions that these hypothetical situations elicit serve as weighty evidence in favour or against the proposed accounts and concepts.


One of the main reasons for the great dependence on thought experimentation in this field is that deciding between the two main approaches to the question of what we essentially are – biological and psychological accounts – requires situations in which bodily and mental characteristics come apart. In real-life settings, an individual’s bodily continuity and the continuity of his or her mental features either occur conjoined or else bodily continuity occurs in isolation, as in a persistent vegetative state. While we cannot learn much from the former case, we do not know how to interpret the latter. The interesting permutation, it appears, is the third one: the presence of psychological features in the absence of bodily continuity. Authors therefore often introduce hypothetical situations that are designed to provide us with this configuration, for the study of which we cannot resort to empirical evidence.


Pioneered by John Locke’s case of the prince whose soul enters a cobbler’s body and his thought experiment featuring the rational parrot, authors have made frequent use of a great variety of hypothetical situations to prove or disprove their respective views about personal identity. We are invited to envisage being teletransported to Mars or existing as mere brains in vats. Such thought experiments are certainly very creative. But are they also suited to act as testing grounds for hypotheses concerning our synchronic and diachronic persistence? What epistemic status can one grant the results that this method delivers?


In this project, I have been investigating the methodological strengths and weaknesses of using thought experimentation to solve problems of personal identity. I found that especially the more fantastical test scenarios that are often employed in an attempt to establish our persistence conditions fail to meet the quality standards of scientific experimental design: objectivity, reliability, and validity. I argued that these standards also pertain to hypothetical experimentation when the imagined situations are so distant from the actual world that their premises become akin to empirical experiments in their own right. Simultaneously, however, the fancifulness of these hypothetical scenarios also precludes proceeding according to these principles.


Many thought experiments are not objective because in imagined worlds that are very different from the actual one, a multitude of uncontrolled variables – rather than only the purposefully manipulated independent variable – exert an influence on the outcome. It then becomes unclear from the modification of which parameters the observed change in the dependent variable originates.


Some fantastical thought experiments have yielded results that are interpersonally inconsistent. They are therefore not reliable. This is so because the more distant a possible world is from the actual one, the more non-ceteris paribus conditions demand specification. Although some situations are so bizarre that full accounts of the envisaged world’s features would assume the length of books, descriptions of the imagined setups usually do not exceed a few paragraphs. It is therefore the subject carrying out the respective thought experiment who must fill in these gaps, which makes the obtained result dependent on certain individual characteristics of the experimenter – an influence that is to be avoided in science at all cost.


Finally, the more distant a hypothetical scenario is, the less likely does it become that the conclusions drawn on the basis of the laws and concepts that obtain in the imagined setup are applicable to our world as it is. When asking questions about personal identity, we are normally inquiring about our persistence conditions. Conclusions derived from fantastical possible worlds therefore run the risk of not being valid when re-applied to the actual world.


In addition of difficulties with possible worlds, I also examined unwarranted background assumptions about the actual world. Many popular thought experiments in the philosophical literature are misleading as they rely on unjustified presumptions about physiological details of the human body. While this is not a feature necessarily inherent to the method of thought experimentation since one could always take into account the available empirical facts, it appears that there has been little enthusiasm in the philosophical community for having the sheer endless options of imaginary setups constrained by anatomical or physiological limitations. Just like physical experiments, thought experiments should as strictly as possible adhere to the standards of good scientific design: to objectivity, to reliability, and to validity – even if this means taking onboard much more empirical data than is customary in philosophy.


Thought experimentation remains a great tool for making scientific progress. In the natural sciences, this method has been pivotal to devising new theories and models. And in philosophy some of the most intriguing exchanges of arguments have sprung from the use of cleverly designed hypothetical situations. It is therefore not the hypothetical method as such that is questionable; nor even is it the hypothetical method applied to questions of personal identity. It is this method combined with overly fantastical scenarios. This is where the conclusions drawn become unreliable or even entirely inapplicable to the actual world.


Associated Publication:

Meier, L. J. (2022): Can Thought Experiments Solve Problems of Personal Identity? Synthese 200 (221): 1-23. DOI: 10.1007/s11229-022-03637-7.

Project 2:

A Novel Solution to Lucretius’ Symmetry Argument

In this project, I suggested a novel solution to a century-old philosophical puzzle. In his only known work De Rerum Natura – a didactic philosophical poem on the world of atoms, on the cosmos, and on human beings – Lucretius famously argued that the periods of pre-vital and post-mortem nonexistence are similar, and that, since we do not regret having failed to exist during the former, nor should we be fearful of the latter. Written approximately 200 years after Epicurus had died, the six books of dactylic hexameters were the first Latin poem in defense of Epicureanism.


The conclusion of Lucretius’ so-called Symmetry Argument is counterintuitive, and there have been many attempts to diagnose what is wrong with it. I proposed a new and different approach to undermining the argument, based on Parfit’s distinction between identity and what matters. When Parfit’s distinction between identity and what matters is applied, not diachronically (as he uses it) but across possible worlds, the alleged symmetry can be broken. Although the pre-vital and posthumous time spans that we could have experienced are indeed analogous with respect to our identity, they are not analogous with respect to psychological continuity, which forms the basis of prudential concern. Lucretius even anticipated the Parfitian distinction. He did not, however, notice the significance that it has for his Symmetry Argument.


Associated Publication:

Meier, L. J. (2019): What Matters in the Mirror of Time: Why Lucretius’ Symmetry Argument Fails. Australasian Journal of Philosophy 97 (4): 651-660. DOI: 10.1080/00048402.2018.1540015.

Project 3:

Testing Lockean Accounts of Personal Identity against the Neurophysiology of the Human Brain

Ever since the publication of John Locke’s Essay Concerning Human Understanding in the late seventeenth century, views that focus on psychological relations have been dominating the debate on personal identity. Many of those who advocate psychological accounts believe, as Locke did, that we persist over time by virtue of being linked to our former selves through a continuity in mental states. An individual x at t1 is regarded as psychologically continuous, and hence, identical, with an individual y at t2 if and only if there is a sufficient number of psychological connections that form overlapping chains between these two points in time. A psychological connection is established by individual x at t1 and individual y at t2 both possessing the same mental state a. This mental state may be an autobiographical memory or any other type of state.


Certain medical conditions can destroy some of these states while the patient remains conscious and able to communicate: individuals who suffer from Alzheimer’s disease or from retrograde amnesia may be unable to recall past events, plans that they had made, or beliefs that they had held. Since it appears absurd to assume that these subjects have ceased to exist and new individuals came into being instead – as Lockean accounts imply – some authors regard memory-affecting conditions as counter-examples to psychological criteria of diachronic identity.


In this project, I investigated whether the opposite permutation also exists: a configuration in which an individual’s mental states persist although his or her brain’s capacity to support consciousness is irreversibly lost. Empirical evidence suggests that this pattern can indeed occur – namely, when the ascending reticular activating system (ARAS), a neural network that originates in the brainstem, is rendered dysfunctional, while the rest of the brain remains intact. This type of damage, I submit, is as great a threat to Lockean accounts of personal identity as are diseases that affect our memories.


The ascending reticular activating system is a neural structure that influences cortical activity to produce cyclical sleep and wakefulness. Put simply, it functions like a controller for consciousness: when its activity level is high, neural oscillations in the cortex desynchronise and the subject awakes; when the neurons of the reticular activating system reduce their firing rate, cortical oscillations synchronise again, and the subject becomes drowsy until he or she finally falls asleep.


The persistence of the neural correlates of a person’s standing mental states, and consequently of the Lockean basis for psychological continuity, is conditional on the cerebrum being oxygenated and supplied with glucose. Conversely, the retention of the capacity for consciousness is dependent on the integrity of two anatomically distinct loci: the cerebrum, which contributes the awareness component to consciousness, and the reticular activating system, which is responsible for wakefulness and originates in the brainstem. Since wakefulness is a precondition of awareness, the destruction of the reticular activating system alone results in the permanent loss of consciousness – affected patients become irreversibly comatose. However, due to the fact that the neural correlates of the mental states that constitute an individual’s long-term memory do not overlap with the core area of the reticular activating system, psychological continuity remains unaffected by damage to the latter. The Lockean condition of a person’s diachronic existence is therefore still fulfilled – although it is obvious that, devoid of the structural prerequisites of wakefulness, the subject must have ceased to exist.


Consequently, damage to this brainstem structure engenders a situation that is incompatible with Lockean accounts as they are currently formulated. I therefore suggest how psychological views of personal identity could be modified to account for the neurophysiological characteristics of the human brain.


Associated Publication:

Meier, L. J. (2023): Memories without Survival: Personal Identity and the Ascending Reticular Activating System. The Journal of Medicine and Philosophy. DOI: 10.1093/jmp/jhad028