People reason not just in beliefs, but also in intentions, preferences, and other attitudes. They form preferences from existing preferences, or intentions from existing beliefs and intentions, and so on, often facing choices between rival between rival conclusions. Building on Broome (2013) and Dietrich et al. (2019), we present a philosophical and formal analysis of reasoning in attitudes with or without facing such choices. Reasoning in attitudes is a mental activity that differs fundamentally from reasoning about attitudes, a form of theoretical reasoning by which one discovers rather than forms attitudes. Reasoning in attitudes has standard format features (such as monotonicity), but is indeterministic (reflecting choice in reasoning). Like theoretical reasoning, it need not follow logical entailment, but for different reasons related to indeterminism. This makes reasoning in attitudes harder to model logically than theoretical reasoning.