   While working in this way, and attempting to define some expressions in terms of others, Leśniewski came to focus on singular propositions of the form ‘\(A\) is (a) \(b\)’, which he henceforth wrote as ‘\(A \isa b\)’, borrowing from Peano the lower-case epsilon, the first letter of the Greek ‘εστι’, is. It was this connection with meanings of ‘is’ that prompted Leśniewski to name this system ‘Ontology’. He thought it would be possible to base it on singular inclusions like ‘\(A \isa b\)’ alone, in addition to concepts taken from the logic of connectives and quantifiers. The key thought was that the following should be true