In a new paper that just appeared in Neuron, Valentin Wyart, Anne-Dominique Devauchelle, Etienne Koechlin and Jan Drugowitsch identify mental probabilistic inference as the main source of behavioral variability. This is in contrast to previous reports that have focused on sensory noise or stochastic action selection. Having identified inference as the main variability source allowed us to leverage this variability to gain further insight into the structure of the mechanisms implementing this inference.