The idea of representing classes and their relation expressed in a proposition by use of circles was originally developed by Euler, a Swiss mathematician of eighteenth century. But its subsequent development and refinements are due to Venn, a British logician belonging to nineteenth century.
A Proposition is said to have existential import if and only it is used to assert the existence if objects of any sort. For example, when we say, 'Some politicians are scholars' we mean that mere exists at least one politician who is a scholar.
There are seven types of possible relation between propositions. Any two propositions will be related in any of the seven ways. These relations are (i) Independence, (ii) Equivalence.
Propositions may be classified into various classes according to different principles, such as the principles of composition, relation, quality, quantity, modality, significance etc.
There are four standard forms of categorical propositions such as A, E, I and O-propositions having the structure of the form, 'All S is P' 'No S is P’, 'Some S is P' and 'Some S is not P' respectively.
As we have seen that there are four types of categorical propositions namely A, E, I and 3-propositions having respectively the logical structure of the form "All S is P", "No S is P", "Some S is P" and "Some S is not P".