 Why should it be the case that simple models have less black-box character than others? One explanation appeals to our cognitive limitations. We can only hold a couple of steps of an argument in our mind at once. Modern, high-level calculations can take hours or days to compute using fast computers. Even if every step was made explicit by the computer, it would be impossible to hold the calculational steps in mind and hence hard to understand the reason for the result, even if one was convinced that the answer was correct. Paul Humphreys has called this the epistemic opacity of simulations (2004).