¹⁵Reducing uncertainty as measured by entropy can even be seen as a general learning principle for the brain (Friston, 2010), and thus failure to reduce uncertainty should generate an error signal. At the same time, reducing uncertainty, unpredictability, and uncontrollability is very closely related to the goal of curiosity discussed in Chapter 6: Uncertainty can be reduced by a curious investigation of new aspects of the environment, and uncontrollability can be reduced by trying out the effects of actions in new circumstances. Another interesting point is that the estimates of uncertainty etc. are not exact either; the estimate of, say, entropy has some uncertainty (estimation error) as well. However, considering uncertainty of uncertainty would lead to a potentially endless recursion, and may not be very useful or feasible.