The proposition must be single.  If it be double, you have what the lawyers call “a squinting argument,” that is, an argument which looks in two directions at the same time.  For example, the proposition, “Commission government would be a good thing for Wytown, but the initiative and referendum are wrong in principle,” involves two separate and unconnected principles, since commission government as first embodied at Galveston does not include the initiative and referendum.  Many people, including those of Galveston and other places in Texas, would accept the first half of the proposition, and disagree with the second half.  On the other hand, “Wytown should adopt a commission government on the Des Moines plan,” would not be a double proposition, though this plan includes the initiative and referendum; for the proposition makes the issue that the plan should be adopted or rejected as a whole.

In some cases a proposition may be grammatically compound, and yet carry a single assertion.  “Municipal government by commission is more economical and efficient than municipal government with a mayor and two chambers,” is really a single assertion of the superiority of the commission plan of government.  In this case there is no danger of getting into a split argument; but even here it is safer to reduce the proposition to one which is grammatically single, “Municipal government by commission has proved itself superior to municipal government with a mayor and two chambers.”  A predicate wholly single is a safeguard against meaning two assertions.

The proposition must not be so abstract or vague in terms that you do not know whether you agree or disagree with it.  Macaulay summed up this difficulty in one of his speeches in Parliament: 

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Surely my honorable friend cannot but know that nothing is easier than to write a theme for severity, for clemency, for order, for liberty, for a contemplative life, for an active life, and so on.  It was a common exercise in the ancient schools of rhetoric to make an abstract question, and to harangue first on one side and then on the other.  The question, Ought popular discontents to be quieted by concession or coercion, would have been a very good subject for oratory of this kind.  There is no lack of commonplaces on either side.  But when we come to the real business of life, the value of these commonplaces depends entirely on the particular circumstances of the case which we are discussing.  Nothing is easier than to write a treatise proving that it is lawful to resist extreme tyranny.  Nothing is easier than to write a treatise setting forth the wickedness of wantonly bringing on a great society the miseries inseparable from revolution, the bloodshed, the spoliation, the anarchy.  Both treatises may contain much that is true; but neither will enable us to decide whether a particular insurrection is or is not justifiable without a close examination of the facts.[4]