@ARTICLE{konikowska87, author = {B. Konikowska}, title = {A formal language for reasoning about indiscernibility}, journal = {Bulletin of the Polish Academy of Sciences}, year = {1987}, volume = {35}, pages = {239--249}, abstract = { Fix $m$ and $n$, and consider structures consisting of a set of $n$ entities and a set of $m$ properties, and for each set $P$ of properties an equivalence relation $\roman{ind}(P)$ on the set $E$ of entities (thought of as indiscernibility with respect to properties in $P$; thus $\roman{ind}(P\cup Q)= \roman{ind}(P)\cap\roman{ind}(Q)$, and $\roman{ind}(\varnothing)$ is the universal relation on $E$). Now consider a language having expressions denoting each set of properties (using the Boolean operations) and whose formulas denote subsets of $E$. The formulas include constants denoting the singleton subsets, are closed under the Boolean operations, and if $F$ is a formula denoting the set $S$ of entities and $A$ is an expression denoting the set $P$ of properties then $\roman{ind}\sb *(A)F$ denotes the set $\{e\in E\:(\forall f)$ (if $(e,f)\in\roman{ind}(P)$ then $f\in S)\}$, and $\roman{ind}\sp *(A)F$ denotes $\{e\in E\:$ $(\exists f)((e,f)\in\roman{ind}(P)$ and $f\in S)\}$. A formula is valid if it always denotes the whole set $E$. The author obtains an axiomatization of the valid formulas. } }