  In this context, a ground model is a classical model M = &lt;D, I &gt; of the language L: we start with a domain of discourse, D, and an interpretation of the predicate F and the relation symbol R. We would like to extend M to an interpretation of the language L + G. So, in this context, an hypothesis will be thought of as an hypothesized extension for the newly introduced concept G. Formally, a hypothesis is simply a function h : D → {t, f}. Given a hypothesis h, we take M+h to be the classical model M+h = &lt;D, I′ &gt;, where I′ interprets F and R in the same way as I, and where I′(G) = h. Given a hypothesized interpretation h of G, we generate a new interpretation of G as follows: and object d ∈ D is in the new extension of G just in case the defining formula A(x,G) is true of d in the model M+h. Formally, we use the ground model M and the definition D to define a revision rule, δD,M, mapping hypotheses to hypotheses, i.e., hypothetical interpretations of G to hypothetical interpretations of G.  In particular, for any formula B with one free variable x, and d ∈ D, we can define the truth value ValM+h,d(B) in the standard way. Then,