What We know from recent research that when people trust each other less, they start to think and behave differently. Many of us would not be surprised to hear this, because it seems to us rational to change the way one interacts with other people if one ceases to trust them. This project is about explaining this relation between trust and rationality. The overarching explanation comprises two elements. The first is that what it is rational for us to do is determined by our knowledge. The second is that whether or not we trust other people can help determine what we can know about them. Together, these two elements can explain why trust affects people's attitudes and behaviour in the way that empirical research suggests they do. Why The project is important for two reasons. First, we know that people's behaviour tends to change for the worse when they become less trusting. Low-trust societies tend to engender corruption, political polarisation, a perceived lack of institutional authority, and the list goes on. Second, we know that in much of the developed world, levels of trust in both people and institutions are falling, and have been for at least 15-20 years. We have clear interest in bucking that trend, To do so, however, we need to understand why trust affects people's behaviour in the way that it does, and part of this project is to understand the relation of trust to rationality. That is what this project is about. How A central methodological aspect of the project is to further explore a phenomenon known as "pragmatic encroachment" which has been much discussed in philosophy recently. Roughly, the idea is that non-truth related factors like risk or practical interests can affect what one knows. For instance, if the possible side-effect of vaccine A is death and the possible side-effect of vaccine B is a rash, some have argue that it will be much harder to know that vaccine A is safe than that B is. The project will explore the extent to which trust can be said to have a similar effect on knowledge, how such account should be spelled out in further detail, and how it coheres with existing empirical data on how people make ascriptions of trust, knowledge, and rationality. SSR The most pressing challenges to the future of human societies require collective decision and action. Global warming, migration, pollution, depletion of resources, etc., are problems that no individual person can solve. Courses of action to remedy these problems are thus necessarily distributed across time, place, and individuals, often with significant burdens to bear for contributors. In many such cases, distributed cooperation can be sustained only on the back of robust relations of trust between contributors and their respective representative institutions. By explaining how trust affects the rationality of choices, the project thereby aims to identify and diagnose both existing and potential barriers to the solution of these problems.