Also familiar is epistemic emergence, the inability to predict the properties or behaviors of wholes from properties and behaviors of the parts. Epistemic emergence can arise as a result of ignorance, such as failing to recognize a relevant variable, or from failing to know how different variables interact in complex networks. It might also result from limitations on human cognitive abilities or in current-generation representational tools (Bedau 1997; Boogerd 2005; Richardson and Stephan 2007). The practical necessity of studying mechanisms by decomposing them into component parts raises the epistemic challenge of putting the parts back together again in a way that actually works (Bechtel 2013a).