It is not self-evident that political parties are a good thing.  The ancient Greeks would, I think, have called them ‘factions’, and considered it self-evident that they were a bad thing: a cause of disunity for the city, and therefore of instability and weakness.

Let us consider the possibilities.  Two parties must either differ or not differ on what are called matters of principle.  Now, if they do not differ on principles, but only about minor or passing matters, for example, about what the national anthem should be, or whether a minimum price should be assigned to bread this year, or whether we should enter a temporary trading or military alliance with another state, this does not seem sufficient to justify the establishment of separate parties.  A party is a stable body, established for an indefinite future, and so cannot be based on an agreement about something transient; it is a body which claims to hold opinions that peculiarly fit it for office, and so it cannot be united by agreement on something minor.

Therefore, they differ in their principles.  This must be either because their principles are incompatible, or else simply because they pertain to different areas of life.  If the principles of the two parties are not incompatible, but simply pertain to different areas of life, then there is as such no reason why they should be two parties and not one.

If the principles of the two parties are incompatible, then these principles must be either eternal truths, for example, on whether natural law is the basis of positive law, or what the contents of natural law are, or whether it is per se desirable that all citizens have some share in the governance of the state; or else they may be principles which go to constitute this polity, for example, whether such-and-such a dynasty should reign, or whether one region of the country should become independent, or how the principal offices are to be assigned. 

If they differ about eternal truths, then at least one of the parties has false principles: and since to possess and seek to propagate false principles is a bad thing, it would be to that extent good for this party not to exist.

If they differ not about eternal truths but about great and abiding matters of state then it seems that the Greeks were right, and parties are a cause of grave disunity.  If the Blues are loyal to the reigning dynasty, while the Greens look to the prince over the water, how does that not harm the commonweal? 

If political parties have nevertheless come to be seen as a normal part of life in a free, democratic and law-governed state, this is, I presume, due to the fact that in the 19th century, the two-party system in Great Britain allowed the country to make changes peacefully which elsewhere were effected through revolutions and civil war: the widening of the franchise and the partial secularisation of society.  The existence of one party committed more to a general ideal of ‘stability’ rather than to any very definite theory of society, and of another party committed to the goals just mentioned, meant that these goals could be obtained slowly and securely.  Trollope in one of his novels has a character (was it Plantagenet Pallister, the young Whig Duke of Omnium?), sketch just such a theory of British political life.  Both through its empire and through the bloody counter-examples of its neighbours, the British two-party system came to seem like a model to be followed.

It may seem that one role for political parties remains: keeping watch over the competence and probity of the executive.  This role is suggested by the title of “Her Majesty’s Loyal Opposition”, given to the largest party in the Westminster Parliament which is not in office.  This function is essential, yet apparently it does not require a separate party; indeed, insofar as a separate party possesses different principles from that to which the ministers belong, it will desire the ministers to be incompetent, rather than competent, in applying them.  There seems no reason why select committees appointed by the sovereign should not fulfil this task.