The unity of the proposition

How is a proposition a single unity? If the sentence can be broken up into subject and predicate, and if subject names object while a predicate names a concept, what is it that relates subject to predicate and object to concept? And what to do with Frege’s problem- that there can be no distinction between concept and object, since, the concept horse can refer as a term to an object? In response to the problem of the unity of the proposition, Wittgenstein held that the elements of a proposition “hang together”, just as the things in a picture relate to one another. A picture of a cat on a mat relates the things cat and mat. Likewise, a proposition is like a picture, depicting some particular situation which can be true or false- e.g. that Socrates is wise.