 But what defines constitutive relevance? Inspired by interventionism about causality, Craver argued that it is best defined by the kinds of interventions that are used by biologists in order to find out whether some entity and an associated activity are part of a mechanism: by certain kinds of experiments. In particular, there are two kinds of so-called interlevel experiments the combination of which establishes (and defines) constitutive relevance. In the first kind, an intervention is performed on some part and an ensuing change is observed in the phenomenon under study. To return to our synaptic example, a calcium antagonist may be used to show that preventing the binding of calcium ions to their receptors prevents the release of neurotransmitter. This is a bottom-up experiment. The second kind of inter-level experiment intervenes on the phenomenon as a whole in order to see some change in the parts. For example, stimulating synaptic transmission by increasing the rate by which action potentials arrive at the terminal will lead to a measurable increase in calcium influx at this terminal. This can be done, for example, by asking a subject to perform a cognitive task (such as trying to memorize something) and by observing changes in calcium concentration by functional magnetic resonance imaging (fMRI). Thus, mechanistic constitution is defined by the mutual manipulability of the parts of mechanisms and the mechanism as a whole.