5.60. A distinction should be made between uncertainties in the value of a variable due to its random variability, called aleatory uncertainties, and uncertainties due to lack of knowledge, called epistemic uncertainties. The main reason for distinguishing between these two types of uncertainty is that, although they are usually treated analogously in modelling, the possibilities and approaches to quantify and reduce these uncertainties are different. Aleatory uncertainties can, in principle, be quantified objectively on the basis of measurements and can be described with probability distributions. Quantification of epistemic uncertainties is always subjective and may be difficult or, in some cases, even impossible. Unlike aleatory uncertainties, epistemic uncertainties are sometimes (although not always) amenable to reduction by further research. In some situations, it may be useful to assign probabilities also to epistemic uncertainties, so that their effects can be studied. However, these probabilities have to be distinguished from those associated with aleatory uncertainties because of different approaches for their quantification and because of different possibilities for the reduction of epistemic uncertainties.