 If theories are underdetermined by observational data, then one is often advised to choose the simplest theory compatible with the evidence (Foster and Martin 1966). Simplicity may be an aesthetic criterion of theory choice (Kuipers 2019), but it may also have a cognitive function in helping us in our attempt to understand the world in an “economical” way. Ernst Mach’s notion of the economy of thought is related to the demand of manageability, which is important especially in the engineering sciences and other applied sciences: for example, a mathematical equation can be made “simpler” by suitable approximations, so that it can be solved by a computer. Simplicity has also been related to the notion of systematic or unifying power. This is clear in Eino Kaila’s concept of relative simplicity, which he defined in 1939 as the ratio between the explanatory power and the structural complexity of a theory (for a translation, see Kaila 2014). According to this conception, progress can be achieved by finding structurally simpler explanations of the same data, or by increasing the scope of explanations without making them more complex. Laudan’s formula of solved empirical problems minus generated conceptual problems is a variation of the same idea.