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Factions can arise when the sovereign power is “a great assembly,” and these are unlawful conspiracies.
If the sovereign power be in a great assembly, and a number of men, part of the assembly, without authority consult a part to contrive the guidance of the rest, this is a faction, or conspiracy unlawful, as being a fraudulent seducing of the assembly for their particular interest. But if he whose private interest is to be debated and judged in the assembly make as many friends as he can, in him it is no injustice, because in this case he is no part of the assembly. And though he hire such friends with money, unless there be an express law against it, yet it is not injustice. For sometimes, as men's manners are, justice cannot be had without money, and every man may think his own cause just till it be heard and judged.
Hobbes, Lev XXII 30
Religious factions are unjust, just as private armies are unjust or illegal.
In all Commonwealths, if a private man entertain more servants than the government of his estate and lawful employment he has for them requires, it is faction, and unlawful. For having the protection of the Commonwealth, he needeth not the defence of private force. And whereas in nations not thoroughly civilized, several numerous families have lived in continual hostility and invaded one another with private force, yet it is evident enough that they have done unjustly, or else that they had no Commonwealth.
And as factions for kindred, so also factions for government of religion, as of Papists, Protestants, etc., or of state, as patricians and plebeians of old time in Rome, and of aristocraticals and democraticals of old time in Greece, are unjust, as being contrary to the peace and safety of the people, and a taking of the sword out of the hand of the sovereign.
Hobbes, Lev XXII 31-2