Casual Introduction (cont.)
Figure 3 returns to the situation in Figure 1, but this time interpolates a new quality specifically tailored to account for the relation between Figure 1 and Figure 2.
This new quality, 
is an example of a differential quality, since its absence or presence qualifies the absence or presence of change occurring in another quality. As with any other quality, it is represented in the venn diagram by means of a “circle” distinguishing two halves of the universe of discourse, in this case, the portions of 
outside and inside the region 
Figure 1 represents a universe of discourse, 
together with a basis of discussion, 
for expressing propositions about the contents of that universe. Once the quality 
is given a name, say, the symbol 
we have the basis for a formal language specifically cut out for discussing in terms of 
This language is more formally known as the propositional calculus with alphabet 
In the context marked by and 
there are just four distinct pieces of information which can be expressed in the corresponding propositional calculus, namely, the constant proposition 
the negative proposition 
the positive proposition 
and the constant proposition 
For example, referring to the points in Figure 1, the constant proposition 
holds of no points, the negative proposition 
holds of 
and 
the positive proposition holds of 
and 
and the constant proposition 
holds of all points in the sample.
Figure 3 preserves the same universe of discourse and extends the basis of discussion to a set of two qualities, 
In corresponding fashion, the initial propositional calculus is extended by means of the enlarged alphabet, 
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