 Moreover, even within one mechanism, the question whether two items are at the same or different levels often has no well-defined answer. This is due to the fact that levels are defined solely in terms of direct part-whole (or component-mechanism) relationships. For example, NMDA receptors and synaptic vesicles are components of the cellular LTP mechanisms, and thus can be said to be at the same level. The components of NMDA receptors in turn include things such as glutamate binding sites and glutamate ions, whereas the components of synaptic vesicles include things such as transport proteins. However, as glutamate binding sites and transport proteins are neither components of one another nor direct components in the same (sub)mechanism, they are neither at the same level nor at different levels (Bechtel 2008: 147). The question how they are related levels-wise has no answer in the mechanistic framework. What this means is that even within a specific mechanism, levels of mechanisms do not form horizontal layers that span across the mechanism, but rather small islands or spotlights formed by the working parts of a (sub)mechanism. Another implication is that even identical things (e.g., NMDA molecules) in the same mechanism are often not at the same level (Eronen 2013). Recently Craver (2015) has argued that the whole idea of being “at the same level” is in fact unimportant or even meaningless in the context of levels of mechanisms; it is sufficient that there is a clear sense in which mechanisms are at a higher level relative to their components (and the components are at a lower level relative to the mechanism as a whole), and this does not require that the components or mechanisms also form horizontal levels. 