Naturalising Rationality

How to make rational decisions under uncertainty? For instance, how to invest money in the stock market given the uncertainty about risks and rewards? The orthodox answer given by economists is to make decisions that are optimal in the light of some abstract decision-theoretic principles known as expected utility theory. The project Naturalising Rationality will take a different approach by taking into account insights from the empirical sciences in order to address the problem of decision making under uncertainty. Psychologists have argued that in decision problems under uncertainty simple heuristics that ignore some of the available information can do better than orthodox decision theory. Similarly, biologists have been concerned with decision rules that perform well in particular environments. The project will make these empirical approaches to rationality subject to close philosophical examination. The project is structured into two parts. The first part will examine the psychological literature on heuristic decision making. Exploring conceptual issues associated with heuristic decision theories is not only of interest from the perspective of philosophy where the notion of rationality figures prominently in epistemology, decision theory and the philosophy of science but has also important practical implications due to the use of heuristic decision making in diverse fields such as medicine and finance. The second part of the project will turn to biology and address the question to what extent principles of rational decision making can be derived from evolutionary biology. The project will make three novel contributions. First, the project will address some unresolved philosophical issues pertaining to the normative status of heuristic accounts of decision making. Second, the project will expand the philosophical literature on the evolutionary foundations of rationality. And third, the project will deepen the link between the psychological literature on heuristic decision making and the philosophical and biological literature on the evolutionary justifications of rationality.

A person presents to his doctor with symptoms consistent with bacterial pneumonia, but also coughs up blood. Coughing up blood is an unusual, though well-known, complication of bacterial pneumonia. However, coughing up blood is also associated with having an airway tumor. In order to account for these observations, two possible explanations are considered. The simple diagnosis asserts that the patient has bacterial pneumonia. In contrast, the complex diagnosis asserts that the patient has bacterial pneumonia and an airway tumor. A principle called 'diagnostic parsimony' suggests choosing the simple over the more complex diagnosis. How can such a choice be justified? In my research I explore a number of possible justifications for this diagnostic principle. I argue that diagnostic parsimony should be considered as a heuristic for medical decision making and supplement conditions under which the choice of this decision tool can be rightfully applied in medical diagnosis.