 The idea that reference to concepts is required by any appropriate explication of the reduction-predicate is also implicit in Nagel’s work (Nagel 1961): Nagel describes bridge-laws as being a posteriori, thereby opposing any sort of analytic reductionism. It has been argued that this might be crucial for fully understanding the concept of reduction. If so, then reduction turns out to be a four-place relation, involving not only entities but also the concepts by which they are picked out or referred to. Truth conditions for sentences of the form ‘a reduces to b’, where the expressions ‘a’ and ‘b’ designate properties, events, phenomena or the like, are sensitive not only to what ‘a’ and ‘b’ designate but also to how they do so, i.e., to the concepts or modes of presentation ‘a’ and ‘b’ express (van Riel 2010, 2013 &amp; 2014; Jenkins 2011).