Because every speech must contain two elements one indicative, the other symbolic, there is a logical foundation for the separation of a proposition into a subject and predicate. The subject is the part containing the indicative constituent, and from which it cannot conveniently be detached, while the predicate is the part purely symbolic.
http://www.commens.org/dictionary/entry/quote-division-iii-substantial-study-logic-chapter-vi-essence-reasoning-5